This release provides both general and specific information on the use of Finite optics on Infinity systems and offers guidance for Finite lens selection. It also explains basic optical factors contributing to success or failure in such efforts and gives examples where the migration to Infinity use can results in new, often very useful lens characteristics. Recommendations and application guidance are also provided. 09/25/16.
From previously unpublished work by D. J. Jackson
Copyright 2016 by D. J. Jackson, all right reserved.*
* This document is distributed under Creative Commons 4.0 License:
Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International.
You are free to download, copy, print, archive or redistribute this document in any format, as many times as you wish, as long as: (a) the document remains whole, complete and unedited, and (b) you neither charge for nor profit from its redistribution and neither use it or any part of it for any commercial purposes whatsoever.
For details visit: www.CreativeCommons.org

by D. J. Jackson ( 20 July 2016 )
Part I of this note presents examples demonstrating basic ways in which finite optics may be used to extend the capabilities and usefulness of Infinity-type Microscopes. While these were developed and tested for use with AO Series-10 and -110 instruments, the basic principles presented here are generally suited to many other Infinity types as well.
Part II will present additional, more advanced and more flexible methods.
1.1 Introduction
The new Millennium has brought the general acceptance and proliferation of microscope systems based in 'Infinity' optics. This has come about after many decades of use of classic, or ”finite,” equipment.
Understandably, there has been much speculation as to the extent to which the earlier optics may or may not be useable on these new “Infinity” instruments.
Although much of the commentary on this issue has been negative, careful experimentation has shown that “finite” optics can, indeed, function quite usefully on Infinity microscopes – under the proper conditions.
This paper to explains some of the basic issues involved and presents methods by which the
‘migration’ of many older, finite optics to Infinity systems may be accomplished successfully. ( Note that the information provided herein should be viewed less as a strict “recipes” for good results and more as a reference for, and “starting points” for, user experimentation.
)
The main reason why unguided attempts at using ‘finite’ objectives on Infinity stands fail to produce any useful results is that the shift to Infinity use alters the fundamental operating conditions for the objective, thereby typically rendering it unsuited for its originally-designed use, but perhaps making it suited for one or more new uses.
While there may be any number of optical parameters involved by far the most important is what is commonly referred to as, “cover glass correction.” This defines the optimal thickness of a thin glass layer which lies between the specimen and the front of the objective and is determined by the optical design of the lens. For Biological objectives this is normally 0.17mm
(or 0.18mm, for some US-made types), and for Metallurgical and some other types. is typically “0mm” (also, “No cover” or “NCG”).
Understanding Cover Glass Correction
The important thing to remember about this is that this parameter becomes increasingly important as the “NA” increases. In practical terms, it isn’t much of a concern for objectives of 10x or less (NA < 0.25) but starts to become important for mid-range objectives (e.g: 20x-25x and NA =0.40-0.50). For objectives of about 40x (NA ~ 0.65) it can impair image quality even if the thickness is off by as little as 0.05mm! And, for higher-performance objectives, there is often an “adjustment collar” to enable setting to lens to match the actual cover glass thickness, often to within 0.01mm.
Now, the importance of all this is that when the cover glass thickness falls outside the range that is best for the selected objective, the resulting image quality will suffer. And the further outside that range, the worse the image will be. And, with that said, we can now explore the “secret” to using ‘finite’ objectives on Infinity ‘scopes – basically, the switch to Infinity focus changes the rules of the game!
In other words, due to this effect, unless you know what’s going on, you’ve lost the race right at the start ...
-1-

1.2 How the Game Changes
Hand-in-hand with the matter of ‘cover glass thickness’ is the issue of Object-to-Lens distance. This is the distance from the actual portion of the object that is in focus to the “optical mid-point” of the objective, through any required cover glass. However, this is also affected by the focus of the objective
(which is why proper “Tube Length” is important with ‘finite’ objectives). Now to obtain the best image it is necessary for all these distances to be correct, including the ‘cover glass thickness’.
Unfortunately, all of this get thrown wildly out-of-whack when we try to re-focus our ‘finite’ objective all the way out to Infinity! And that is basically why unguided attempts at this typically fail.
In fact. there is no easy way to simply re-use a ‘finite’ lens at Infinity – at least not for its origina l use.
( However, there are ways in which to apply such optics to new uses, as we shall soon see.
)
To understand how this works, consider that, as the focus of the objective is shifted toward Infinity, two related changes occur:
(1) the ‘Object-to-Lens distance’ is reduced , and ,
(2) the ‘Cover Glass Correction’ is also reduced .
When the lens is finally focused at Infinity we find we have a new situation, where the best image will be obtained at a new, shorter , Object-to-Lens distance (the “Focus Shift”) and also at a new, thinner, optimal Cover Glass Thickness (the “Cover Glass Correction” Shift). Thus, in many cases, it is then entirely possible to obtain very good image quality , at Infinity – but only under these new conditions!
With this understood, it now becomes less of a technical problem and more of a ‘parts selection’ issue to achieve success with ‘finite’ to Infinity objective migration.
To maximize the likelihood of success we merely need to choose those objectives which will function as required under the new conditions.
And just what are those “ new conditions ”?
Without much difficulty we can define three main “classes” of application conditions which would make the ‘finite’ to Infinity migration process worth the effort:
(1) Low-power Long Working Distance objectives – since most Low-power Infinity objectives have annoyingly short working distances. Longer working distances would potentially permit both specimen manipulation and “top lighting.”
(2) Use Biological objectives for “No Cover” applications – this would eliminate the need for tedious specimen preparation and mounting, and also remove the need to fit a cover glass for even temporary mounts.
(3) Create Objectives for Water Immersion Use – there are very few objectives available which support ‘direct’ water immersion, especially for Infinity ‘scopes. The ability to re-purpose standard objectives for such use can be especially appealing.
In the subsequent sections of this Part we will cover each of these application areas separately and see just how easy it can be to accomplish the migration of older, ‘finite’ objectives to these new uses.
-2-

1.3 Understanding the New Rules
Migration Rule 1 – Reduced Object Distance
The term “Object Distance” is somewhat uncommon in microscopy, but not in Optics. This may be due to the fact that it is a largely theoretical concept and is not easily measured, except, perhaps with special equipment. Nevertheless, it is a valuable concept when it comes to optical design, as well as when one tries to estimate imaging parameters.
However, it is ultimately more useful to rely on the more familiar terms, Parfocal Distance and Working
Distance when trying to communicate the magnitude of these effects, so those terms will be used here.
Parfocal Distance is the distance from the mounting shoulder (nosepiece face) to the object, as measured through any specified cover glass.
Working Distance is the distance from the closest part of the objective to the surface of the object or of the cover glass, if one is specified. It may be estimated as Parfocal Distance, minus the barrel length of the objective.
Now, this shortening of these distances (the change is equal for all three) is a natural consequence of the shift from ‘finite’ focus to Infinity focus for the objective. This is because it is necessary to move the object closer to the lens in order to shift the lens’ image further away (e.g: all the way to Infinity). This is a simple calculation and it provides a result (the change in distance) for all three terms, in one shot!
So, just why is this important? Because, in many instances, the initial Working Distance of the objective may be either very close to or even less than the change in distance. In such cases there is simply no way the objective will be useable at Infinity and it is simply best to move on to another choice. Knowing in advance that something is simply not going to work can be a major time-saver -- and money-saver.
-3-

Migration Rule 2 – Cover Glass Correction
As an objective’s level of refinement (e.g: NA) increases, it becomes increasingly susceptible to any disturbances in the object-to-lens optical path. For objectives designed to function primarily in air (e.g:
“dry”) the cover glass can be a most important consideration in obtaining the best image quality. In fact, many Biological objectives are marked with the optimum cover glass thickness (typ.,“0.17", for
0.17mm). Metallurgical objectives are typically marked, “0", for ‘No Cover’. Low-power objectives may be marked, “–“, for ‘With or Without cover’.
Now, while this change can be very challenging to estimate by calculation it is a rather thing to simple to measure, and so the following table has been determined empirically:
Note that neither the initial Parfocal Distance and Working Distance are relevant – only the Focal Length matters when determining C.G. Shift.
From this basic data it is possible to draw several very general conclusions which may serve as a guide to selecting ‘finite’ objectives for Infinity use: :
(a) For objectives of <10x and NA 0.20 or less , the change may be ignored.
(b) For objectives of 10x with NA of 0.30
or less , the change may be ignored.
(c) For objectives of 20x with NA of 0.40
or less , the change may be ignored.
(d) For objectives of 40x with NA of 0.50
or less , the change may be ignored.
For objectives of 40x with NA of 0.50 to 0.75
, a cover glass should not be used.
(e) For objectives of 50x with NA of 0.50 to 0.75
, a cover glass should not be used.
For objectives of 50x with NA > 0.75
, the NA must be reduced for good images.
(f) For objectives of 60x-63x with NA of 0.70 to 0.90
, a cover glass should not be used and Correction Collar adjustment may be required for good images.
-4-

Migration Rule 3 – Changed Magnification
You may note that the term, " Focal Length " is used here. This is because, when used at Infinity , a finite objective will produce a Magnification determined by its Focal Length – this is loosely related to the magnification inscribed upon it. However, final Magnification depends upon both the Focal Length of the Objective and the Focal Length of the Tube Lens in the Microscope, as follows:
(Inf.)
(TL) /
(Obj.)
Where FL
(TL)
is the Focal Length of the Tube Lens , and FL
(ob j.)
is the Focal Length of the Objective .
This Magnification effect is shown in the table below for a number of common Infinity systems. This comparison is based on using 10x objectives on an AO Series-10 or -110 microscope :
Maker FL( TL ) Mag
( Inf.
)*
Mag.Factor* Remarks
AO
AO
Aus Jena
Leica
Leitz
Meiji
Nikon
Olympus
Olympus
Zeiss
183mm
163mm
250mm
200mm
250mm
200mm
200mm
180mm
180mm
165mm
10x
11x
7x
9x
7x
9x
9x
10x
10x
11x
100 %
112 %
72 %
92 %
72 %
92 %
92 %
100 %
100 %
112 %
Series-10,-110
Series-50,-60.
Non-RMS (
M25 mount thread.
RMS (
M26, 60mm obj.
M25, 60mm obj.
RMS, UIS & UIS-2
RMS, BH-2 (
RMS typ. M19
1950's-70s
Met
)
.)
)
* For listed System’s 10x objective when used on AO Series-10 or -110.
When swapping objectives between Infinity systems, the resulting Magnification Factor may be found by adjusting for the Tube Lens Focal Lengths, as follows:
( target )
( original )
The following discussions will provide specific examples of the proper use of these basic rules, based on specific types of applications, where commonly-available finite objectives may be put to new uses on
Infinity systems.
-5-

1.4
Infinity Applications: ' No Cover' optics
Often it is desirable to be able to examine biological or material specimens without resorting to complex mounting procedures. Often, such " un-mounted " specimens may simply be placed under a coverglass and then examined with conventional objectives. And, at lower powers, even the cover glass may be eliminated.
However, at higher powers, and for many types of specimens, such an approach may give poor results.
This is because the better objectives are designed for use under specific conditions, and, for most
Biological types, this means for use with a cover glass of specific thickness (typ. 0.17mm.) The cover glass thickness forms an important part of the objective’s optical path and simply eliminating it (for
NA’s greater than about 0.45) upsets the objective’s optical corrections.
It is for such reasons that "No Cover" objectives are designed, although these can be costly and choices tend to be limited, especially for Infinity systems. Fortunately, there is a simple alternative -- just use a
'finite' objective instead!
Fortunately, the way this works out is this: for ordinary Biological objectives of about 40x (e.g: 4mm focal length), migration to Infinity will result in their cover glass correction being reduced from 0.17mm
to roughly zero. Thus, such objectives can potentially function as " No Cover " types simply by moving them to an Infinity stand. ( Note that this approach works only for 'dry' type objectives -- Immersion types are much less sensitive, as will be discussed later.
)
Also, the Parfocal Distance is minimally affected -- typically by only about 0.1mm. This makes the older
AO/Spencer objectives (e.g: 43x/0.66) very attractive for such use, as they will be nearly parfocal with usual the AO Infinity types.
Another option is to use the old Nikon-s (short, black base) objectives since the parfocal error there will only be about 0.5mm, easily compensated with a 0.5mm spacer, if desired The Nikon-s 40x/0.75
Fluorite gives excellent results in ‘No Cover’ use, for example, while the Nikon-s 40x/0.65 Plan is yet another option, but offers a Plan image. ( Do not confuse these with the later, longer Nikon CF lenses.
)
The “Thin-cover” Option
There are many occasions when a “quick-and-easy” mounting method is desired, but using a standard cover glass is not an option ( e.g: when using “No Cover” objectives ).
Fortunately, there is a very simple “work around” for such situations: simply use clear
(Scotch-type or ‘cellophane’) tape!
Such tapes are largely optically transparent and have thicknesses in the 0.03 to 0.05mm
range, making them quite close to “no cover” optically. The main limitations are that the specimen should truly be dry and that small, visible air pockets may form where the tape does not fully contact the underlying slide.
The chosen specimen is simply placed on a slide and then carefully covered with a piece of transparent tape, which may then be trimmed as desired. The resulting “Thin-cover mount” may then be examined under medium-to-high magnification.
Suitable specimens tend to be of the “forensic” type (e.g: hair, fibers, mineral granules, etc.) but there are also many possible biological specimens, also (e.g: dry insect parts, mold, pollen, soil samples, etc).
-6-

1.5
Infinity Applications: Low-power & LWD Optics
Now, as you may be well aware, there seems to be a "hole" in the standard objective lineup. Except for the somewhat scarce AO 6.5x/0.2
(Cat. # 3006), there really isn't anything available from AO between
4x and 10x for Infinity! Furthermore, the Working Distance on the standard AO 4x Plan objectives is less than 8mm. ( Note that the non-Plan #103 and #130 economy "Student" lenses are an exception ).
So, what can you do if you need more than 4x, but less and 10x, and/or a decent Working Distance (e.g:
>10mm)? The answer is, as we will soon see, to simply go finite!
Now, going this route you need to understand that, since the lens' focal lengths are quite significant relative to their finite tube lengths (e.g: 160mm) there will be a much greater shift in lens parameters when they are used on an Infinity stand. This means that the Magnification will increase while the
Working Distance (and Parfocal Distance) will both decrease, usually substantially.
As an example, consider the AO 5x/0.14 Achromat (from the AO Series-15/-35). When mounted on an
Infinity stand, the Working Distance goes from 16.7mm down to a more modest 12mm, However, the
Magnification increases from 5x to about 7.5x. So, with this lens, when used at Infinity, you get not only greater Magnification, but also a decent Working Distance (relative to the Infinity AO 4x/0.10 Plan.)
However, it is not necessary to use only AO-Spencer optics, as these can be hard to find. Similar optics are available from other makers (e.g: B&L) and can provide much the same results. Many of these lenses are readily available and may often be found at low cost (e.g: on Flea-bay, etc).
The B&L 3.5x/0.09
is one of these, quite common, and yet it can provide good results on an AO-10 or -
110 Infinity stand. Here, the Working Distance (for Infinity) is a bit more than 9mm and the
Magnification is about 6.3x. But, even better, the B&L 6x 22.7mm 0.17 Achromat yields a Working
Distance of over 15mm and a Magnification of 8x!
B&L made a whole series of basic low-power objectives which can provide good service at Infinity, except for the 28mm. ( This is because the 28mm has a casing that extends just too far forward to allow a useful Working Distance when used at Infinity.
)
Another alternative, largely forgotten, is the B&L 10x/0.25 Divisible objective . Here, using the rear section only , a Working Distance of 23mm may achieved with a Magnification of just a bit over 6x, when used at Infinity.
Test results for a range of common finite low-power objectives are shown on the following page:
“Don’t-care” Objectives
Note that with most of these low-power lenses the NA is low enough (e.g: <0.20) such that Cover Glass correction issues do not arise and so many of these objectives may be used with or without a “cover glass” and even through several millimeters of glass or water and still provide good images. ( See Section 1.3 for more on this topic.
)
Alternatives, including the use of DIN objectives from other makers, are discussed in Part II.
-7-

-8-

1.6
Infinity Applications: Water Immersion
This brings us to the topic of Water Immersion optics. Unfortunately, the term, "Water Immersion" has two very different meanings, and thus can imply two very different optical conditions. First, there is the common " wet mount " configuration where the specimen lies beneath a cover glass. Here, the optical path consists of water plus a thickness of glass. The other configuration omits the cover glass and the objective is directly "dipped" into the water with the specimen (thus the term, “ dipping objective ”).
While each mode has its uses, and its proponents, the point to remember is that the better (higher NA) objectives are usually limited to just one mode or the other.
Water Immersion objectives designed for use with a cover glass typically require some means of adjustment to compensate for variations in the thickness of the glass from the ideal (typ. 0.17mm). For high-NA objectives this usually means an NA>0.75. However, for objectives where the range of glass thickness is great (e.g: for Inverted microscopes, which may view through the bottom of a Petri dish) the
NA may be as low as 0.4, although the thickness adjustment range may be up to as much as 0-2mm.
Now, since the Cover Glass correction change (due to a switch from finite to Infinity) depends upon the
Focal Length (Magnification) of the objective in question, we can see that this results in some real opportunities for the world of Water Immersion. However, the greater the Magnification
First, consider two commonly available ‘finite’ Water Immersion objectives, the LOMO 70x/1.23 Apo and the 85x/1.00 Achromat . Both are intended for use with a cover glass and so feature adjustment collars to set the necessary level of correction. Normally, these are not suitable for Direct Immersion use.
However, through the magic of Infinity migration we find that these lenses can, indeed, function perfectly in a "dipping" mode – simply by using them on an Infinity stand and setting the correction adjustments in the 0.10 to 0.13mm range. ( These setting can be "fine-tuned" by the user to compensate for lens-to-lens manufacturing differences, if desired, )
However, there is also the LOMO 40x/0.75 Achromat , a lower-cost lens in a much more practical magnification, which requires no adjustment.. This lens also functions quite well on an Infinity stand.
And, lastly, there is a LOMO 30x/0.90 Achromat and this yields good images in Infinity use, both with and without a cover glass. (Just be sure you have the Water Immersion versions of these lenses!)
There is one other objective to consider, especially if lower magnification is desired. This is the
Olympus C20x/0.4
( the plain version, Not LWD or Plan ). This has a slightly longer parfocal distance, and also comes in a CPL (phase contrast version) that works quite well in normal and most other common modes (e.g: darkfield or oblique). These objectives are compared in the table below:
Additional information on using Water Immersion with Infinity systems may be found in Part II.
-9-

1.7
More about Finite Systems
In the preceding pages a number of suggestions have been made about using finite optics on the AO-10 and -110 for specialized needs. However, it needs to be stressed that range of finite optics potentially useable on these stand is much less limited than those earlier remarks might suggest.
In theory, the range is nearly unlimited, however, we need to keep in mind that certain very practical limits still exist when choosing “foreign” ( non-AO ) optics. These limitations are:
1.
The AO-10 and -110 have a limited coarse-focusing range – only about 10mm.
This constrains the range of parfocal distances that can be accommodated on any one nosepiece without the need to re-adjust the stage height.
2.
The AO Tube Lens imparts moderate compensation for lateral color in the image.
This means that Color-Free optics will be over-compensated, unless a different body tube is used ( Note: The Reichert/Leica Microstar-IV and Leica ATC-2000 body tubes fit these AO stands and both have a non-compensating Tube Lens.
)
3. For finite objectives of 10x or less, the actual parfocal distance in Infinity use may be considerably more than the “normal’ value listed for that type. ( See Sections 1.5 and
1.9 for additional details on this issue.
)
For the most part, life will be easier with the Series -10 and -110 if DIN-type (45mm) objectives are simply avoided. ( However, for the adventurous sort, information on this issue is included in Part II.
)
Nevertheless, there are still plenty of older ‘finite’ types to choose from, including the following:
Alternative ‘Finite’ Objectives
Maker Parfocal Dist.
AO /Spence r
Aus Jena
Bausch & Lomb
Carl Zeiss (Jen a)
Leitz (W etzlar)
LOMO (Russia)
Nikon-s Series
Olymp us (BH )
Spencer Lens
W ild-Heerbrugg
33.7mm
32 or 33m m
35.7mm
32.0mm
37.0mm
32.0mm
33.5mm
37.0mm
33.7mm
37.0mm
More data for most of these types may be found on the Internet and/or from manufacturer’s literature.
One point not noted above is that many of these lenses have usage limitations, being designed either for use only with a cover glass (e.g: often marked. “ /0.17
"), or use only without a cover glass (typ. Marked
“ / 0 "). In general, the “/ 0" types may not function well in Infinity systems since the shift to Infinity may very well also shift the objective’s correction to a negative value!
( See Section 1.3, above, for more details on this issue.
)
-10-

This release provides additional information on extending the use use of both Infinity and Finite optics on
Infinity microscope optical systems. It also explains additional optical factors contributing to success or
failure in such efforts and gives examples where these new methods can often result in new, and often
very useful lens characteristics. Recommendations and application guidance are also provided. 10/14/16.
From previously unpublished work by D. J. Jackson
Copyright 2016 by D. J. Jackson, all right reserved.*
* This document is distributed under Creative Commons 4.0 License:
Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International.
You are free to download, copy, print, archive or redistribute this document in any format, as many times as you wish, as long as: (a) the document remains whole, complete and unedited, and (b) you neither charge for nor profit from its redistribution and neither use it or any part of it for any commercial purposes whatsoever.
For details visit: www.CreativeCommons.org

by D. J. Jackson ( 17 July 2016 )
Part 1 explored the basic process of simply mounting a ‘finite’ type lens on an Infinity scope. In Part 2 this process is extended by allowing the focus of the objective to be altered by optical means, so as to either optimize the use of the objective or enable new uses. Infinity objectives can also be used with this method. Again, these methods were developed and tested using AO Series-10 and -110 instruments, although the basic principles presented here may also be suited to some other Infinity types as well.
2.1 Beyond Infinity?
Although Infinity is commonly thought of as a sort of “end point,” in reality (especially optically), it doesn’t have to be. If an Infinity objective is moved closer to the Object, then its image will be formed at a distance that is “beyond Infinity.
The diagrams below depict an Objective under the three main conditions of focus:
To form and image in an Infinity System then, another lens must be added to restore the focus to
Infinity. This added lens, termed an “ Image Shift Lens ,” then simply allows the Tube Lens to function normally. However, the Objective is now in a new (closer) position relative to the Object. This results in a change in the performance of the Objective due to these new conditions. And, for and Infinity
Objective, this change is similar to that undergone by a ‘finite’ objective migrated to Infinity use!
The diagram on the following page depicts an Image Shift Lens and its function:
-12-

In the optical world, Infinity is more or less a state, in which light rays travel parallel and thus no ‘real’ image is formed. Since the formation of a “real” image requires the convergence of light rays, light rays that are in an Infinity (parallel) mode may be caused to produce a real image simply by inserting a converging lens. An image is then formed at the focus of that lens.
This is basically how an Infinity microscope works, where the Tube Lens acts as the “converging” lens and results in an image being formed at the Lens’ focus – just in front of the eyepeice(s).
However, consider the case where a second “converging” lens is added to the system, just beyond the objective and some distance before the Tube Lens. This new lens will result in a new image being formed, but one which is inaccessible to the eyepiece(s). The make the image visible, the objective must be focused even closer to the object, and this results in a new condition in the instrument where the light rays exiting the objective are now “diverging,” instead of parallel. ( See diagram, above.
)
While this may seem very odd, at least initially, the situation is rectified by the new, added lens mentioned above. When the objective is now focused properly the “diverging” rays from the objective become parallel ( Infinity focused ) before entering the Tube Lens. Thus, although the objective is now focused “beyond Infinity” the added “Shift” lens corrects this and the microscope functions properly.
So, the question now is, “Why bother to do all this?”
And the answer is, “Because the objective is now operating in a “quasi-finite” mode, and this new approach now allows us to apply our ‘ finite-to-Infinity ’ migration tricks to Infinity objectives !”
What we have created is a technique that allows us to transform normal Infinity objectives into “No cover,’ Water Immersion, Glycerin Immersion, or other types, using methods similar to those used for finite objectives. We can now use objectives designed for out microscope for entirely new purposes!
The details of this process and use in actual applications are covered in the following Sections.
-13-

2.2 Image Shift Lenses
The purpose of an Image Shift lens is, in the present instance, simply to displace the image formed by the objective from Infinity to some finite position. In practice, however, this image is never actually formed. Instead, the microscope objective is re-positioned such that the image-forming rays exiting the
Shift lens are restored to an Infinity focus. Under these conditions the remainder of the microscope functions normally, except that the objective is now in a new position, typically closer to the object.
The reason for undergoing this exercise is simply to allow this re-positioning to occur, since it is this change which alters the functioning of the objective. This mimics what occurs with the finite-to-Infinity migration process, except that here the user has far greater control over the outcome.
Previously, there was only a single step, from ‘finite’ to Infinity operation, with nothing in between. In such cases the quality of the final image was largely a matter of choosing the right objective initially.
Now, there are a range of intermediate steps available, between ‘finite’ and Infinity, as we shall see . . .
Within this method there are two primary variables: (1) the strength (or, Focal Length) of the Image
Shift lens itself, and, (2) the spacing of this lens from the objective. Of these, the first is most critical.
In general, it is best to keep the Shift lens close to the objective – the rear of the nosepiece is usually a good location. Also, it is essential that this lens is located closer to the objective than to the Tube Lens if the method is to work correctly. So, once again, the rear of the nosepiece is a good location.
The AO Series-10 and -110 both have a deep “well” located in the arm, just above the nosepiece. This is easily accessed simply by removing the viewing body, making these scopes a good choice for this implementing method.
The diagram below compares a standard Infinity System with one having an added Image Shift Lens:
-14-

In this diagram, note that the Objective provides a converging focus to the Image Shift Lens and the
Shift Lens simply re-establishes Infinity focus. Not depicted is the fact that the Objective is now in a new position, relative to the Object, which results in a new set of optical conditions. Consequently, we can now set the optical conditions for the Objective simply by the choice of the Shift Lens strength
(Focal Length). We are no longer limited by chance ‘matching’ of the Objective and our requirements.
Now, as for specific choices for Shift lenses. If we wish to mimic a ‘finite’ objective, a Shift lens with a
Focal Length in the range of 150 to 170mm would be a good choice. However, this would also be an extreme case for the method and the resulting image quality would likely be unsatisfactory. You would probably be better off just choosing a ‘finite’ objective to begin with and not even have to bother using a
Shift lens!
More reasonable choices for the Shift lens would be in the range of 300 to 500mm, which would equate
(roughly) to ‘half-finite’ (300mm) and ‘third-finite’ (500mm). For these, the change in objective corrections in the final image would be about one-half or one-third those of a normal ‘finite” objective.
It is in this “intermediate” range where the method works best and is easiest to implement.
Why only “half-way,” or so?
Because the full, ‘finite’ to Infinity change is “overkill” for many situations, especially with mediumpower objectives – the change in objective corrections is just too much to result in a decent image. But, with this new approach, the change becomes much more controllable, to the extent that the method may be easily tailored to produce optimum results for a given application.
Image Shift Lens Example
Consider, for example, the AO 50x/1.00 Plan achromat . This yields good images under oil immersion in normal use. But, suppose we wish to adapt this lens for Water
Immersion use?
If we attempt this directly, simply by switching from oil to water for immersion, we find the resulting image is rather disappointing, and the lens’ (or Condenser) iris must be closed considerably to produce a useable image.
However, by applying the Image Shift method, we find that the same lens can be made to yield quite decent images, with Water immersion, even at full aperture! And this result can be produced simply by placing a ~300mm (+3D.) lens in the rear of the nosepiece!
And the price to be paid for this adaptation? Merely a loss of Magnification of roughly
10%, so the final Magnification, in this mode, is about 44x (vs. 50x in the normal mode).
Additional examples of the use of the Image Shift Lens method are presented in the next Section.
-15-

2.3 Basic Rules for Image Shifting
While the results may be comparable to those obtained by migrating objectives from ‘finite’ to Infinity use, with the Image Shifting technique the Basic Rules are somewhat different. Also, the very flexibility of the Image Shift method can lead to confusion (and poor results) unless some systematic procedure is followed. The Basic Rules for Image Shifting may be summarized as follows:
1.
2.
3.
Basic Rules for Image Shifting – Infinity objectives
The Objective's Parfocal Distance and Working Distance will be reduced .
The higher the objective’s Magnification the less this effect will be, but the overall extent depends on the choice made for the Shift Lens Focal Length. *
Unfortunately, means that some older Plan objectives cannot be brought into focus when used at Infinity. ( The Zeiss 10x Planachromat is one example.
)
The Objective's Cover Glass Correction is reduced and Infinity use retained.
However, the higher the objective’s Magnification the less this effect will be.
The overall extent depends on the choice made for the Shift Lens Focal Length.*
For Immersion objectives this can easily affect the choice of immersion fluid.
( For example, from Immersion Oil to Glycerin, or even from Oil to Water.
)
Both the Objective's Magnification and Focal Length are reduced . The extent of this reduction depends upon the choice made for the Shift Lens Focal Length.* .
* See the tables provided in the Reference Section for guidance on this issue.
For example, one common AO/Spencer Infinity lens worth considering is the 45x/0.66 (Cat. #1116) .
This was a standard lens on the early AO Series-10 models but is an Achromat , rather than a
Planachromat. Still, it can provide very good service, especially when coupled with an Image Shift
Lens. Used with a +3D Shift lens, this objective functions very well in a “No Cover” mode, providing a sharp, nearly flat-field image with good contrast, up to its full aperture. It also has a very good Working
Distance for a 40x, about 0.7mm. In the Image Shift mode, it produces a Magnification of about 45x.
( The use of the Image Shift mode, in this case, compensates for the removal of the Cover Glass from the
Object path.
)
All About “Diopters”
When dealing with Image Shift lenses, we are concerned primarily with simple lenses having relatively long focal lengths – similar to “reading glasses.” It should come as no surprise then that these lenses can share the same descriptive nomenclature – “Diopters”
(or, simply, “D.”), rather than Focal Length. Now, this antiquated convention is not hard to understand, it is merely a “shorthand” for expressing the Focal Length, as follows:
Diopters = 1000mm / [Focal Length] where the lens’ Focal Length is in mm. ( Thus, +3D = 330mm.
)
-16-

A similar lens, but one which yields very different results is the 43x/0.55 (Cat. 132) . This is a “student” lens from the Reichert-Jung Series-160 . ( Do not confuse this lens with the earlier Cat. #114, from the
AO Series-60, as that is a very different lens!
The #114 is a straight, tubular design while the #132 has a very definite “step” between two different barrel diameters, and is much better than the #114 optically.
)
The #132, used with a +3D Shift Lens makes a very good Water Immersion (“Dipping,” no cover glass ), with a very good Working Distance of roughly 0.6mm. Now, although its NA of 0.55 is quite a bit lower than the AO 50x/0.80 mentioned in Section 2.2, this is not necessarily a handicap when dealing with live specimens. When used with the +3D shift lens, the #132 yields a Magnification of about 37.5x, and is capable of a very good image up to its full aperture.
Unfortunately, this lens is considered somewhat “cheap” (it has an aluminum barrel) and is thus may be a bit hard to find, as many users ( at least until this word gets around ) don’t seem to consider it worth selling – in fact, you may well have to buy a whole Series-160 scope just to get one of these lenses!
Understanding Image Shift Results – Good and Bad
At this point it is worth stating that this Image Shift Lens method is concerned primarily with altering the objective’s correction for only a single parameter, namely, “Spherical
Aberration,” most commonly referred to simply as, “Cover Glass correction.”
Unfortunately, this is only one of many parameters that affect the quality of the final image ( however, it is often the most important ). The new result of this is that any one value of Shift Lens can produce different results when used with supposedly “similar” objectives, due to the existence of the other parameters. When everything moves in the proper direction with a particular change, an excellent image can result. In other cases, a really good image may simply prove elusive, no matter what value is chosen for the Shift
Lens. And this is why this Method may not always produce the results we hoped for.
The use of the Image Shift Lens method with ‘finite’ objectives well be discussed more fully in the following Section.
-17-

2.4
Image Shifting – Finite objectives
As an initial example for this Section, we shall return to ‘immersion’ objectives. The case in point here is the excellent E. Leitz (Wetzlar) 54x/0.95 Fluorite Oel ( 37mm version ). This is a “170mm” lens with an excellent reputation for image quality. However, it is also not a Plan lens, which is probably just as well, since that fact helps to keep the price down to reasonable levels (usually). The real issue with this lens is that it has a relatively-high NA (0.95) and it also lacks an iris.
Now, the lack of an iris means that the lens’ NA is fixed at a high value, which would make it quite sensitive to cover glass thickness – if it was a “dry” lens. ( However, as an immersion lens we have much less to worry about as these are much less sensitive to that parameter.
)
So, it should not surprise us that this lens, in Image Shift mode, performs similarly to the AO 50x/0.80, mentioned in Section 2.2. Using a +3D Shift Lens , this objective provides excellent images with Water
Immersion ( No cover ). And, without any Shift Lens, it can also provide very good images with Water
Immersion using a cover glass. Without a cover glass, the Working Distance is about 0.3mm.
Also, there are significant image differences – the AO 50x is a Plan lens, but an achromat, while the
Leitz 54x is a Fluorite lens, but probably best considered as a ‘ flat-field ’ type. Thus, the AO may be expected to provide sharp coverage over the whole visual field while the Leitz may be expected to suffer increasing ‘softness’ in the outer areas of the field. Yet, the color fidelity, sharpness and contrast of the
Leitz are simply amazing (barring lens defects), even though the AO is still quite good.
Now, another fine lens is the E. Leitz (Wetzlar) 25x/0.65 Apo ( 37mm version ) – truly, an excellent medium-power objective. But, again, the high NA of this lens makes it potentially quite sensitive to cover glass correction. Fortunately, for use dry, without a cover glass , this is one of the very few Apo’s which function well with no Image Shift !
However, if you choose to use this lens on specimens that are under a cover glass, the image will become rather poor! This is because the cover glass correction is, under those conditions, just totally wrong. Fortunately, this situation can be corrected via the Image Shift method, BUT, only by using that in a new way – with Negative Shift . In this new mode, the shift lens must be a negative lens, in this case we need to use a “-3" Diopter lens (-330mm focal length). Using this lens in our Image Shift setup then restores the image quality by changing the cover glass correction to something more appropriate.
Negative Lenses
Unfortunately, Negative lenses don’t “grow on trees” and can be quite difficult to find at any decent price, or, even at all! The best source of such lenses now seems to be the old
Kodak “Telek” lens series, which offered negative lenses in -1, -2, -3, and -4 Diopters.
These are typically provided in the old, threadless Series ‘filter’ mounts.
The point here is that even though the Image Shift method can get you the performance you want in one particular mode, that doesn’t mean you can translate your success into any alternative modes, The more refined the objective, the more demanding it will be in terms of achieving proper cover glass correction for its current use.
-18-

This provides general information on the use of Finite optics on Infinity systems and offers basic criteria for
Finite lens selection. It also reveals basic optical factors contributing to success or failure in such efforts and
gives examples where the migration to Infinity use can results in new, often very useful lens characteristics.
[
]
[
]
From previously unpublished work by D. J. Jackson
Copyright 2016 by D. J. Jackson, all right reserved.*
* This document is distributed under Creative Commons 4.0 License:
Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International.
You are free to download, copy, print, archive or redistribute this document in any format, as many times as you wish, as long as: (a) the document remains whole, complete and unedited, and (b) you neither charge for nor profit from its redistribution and neither use it or any part of it for any commercial purposes whatsoever.
For details visit: www.CreativeCommons.org

Version: 1c Released: 21 Sept. 2016
1. The Finite > Infinity transition
It seems that. more and more these days, the question is raised, "Can I use Finite lenses on my Infinity 'scope?"
However, the better question might be, “Which Finite lenses might work best on my scope?”
Now, while random efforts at this seem too often to yield unsatisfactory results, it has been demonstrated that such
Finite > Infinity "transition" efforts can be made successfully and very useful result achieved!
Unfortunately, as is usually the case, there is a "price to be paid" for accomplishing this -- most frequently some minor loss in resolving power. However, this is not always the case as there are some lenses which can gain new capabilities if and when the "transition" is properly managed. ( Such "special cases" will be covered separately, in detail, later in this document.
)
In general, the transition of a lens from Finite to Infinity use results in two significant changes to its characteristics:
(1) there will be a Reduction of Working Distance , due to the transition to Infinity focus, and ,
(2) there will be a Reduction in the Cover Glass Thickness for which the lens works best.
The ' Reduction in Working Distance ' is a function of the Focal Length (Magnification) of the lens -- the longer the
Focal Length, the greater the Reduction will be. (Note, however, that with some lens designs, the shift to Infinity use will cause the Working Distance to drop to zero, or even less, making the lens impossible to focus. This is common with some older low-power plan designs, usually of 10x or lower. Simpler lenses, fortunately, rarely exhibit this problem.)
The ' Reduction in Cover Glass Thickness ' is a function of the Effective NA of the lens -- which is a combination of both the Objective's NA (as rated) and the Condenser's NA, as set by the Condenser Iris. Consequently, the
Condenser Iris becomes a valuable tool in achieving optimal functioning of the objective following the transition to
Infinity focus!
In general ( for Dry objectives ), a Finite > Infinity shift will result in a loss of Working Distance and a need to use either a thinner cover glass, or none at all, depending upon the focal length of the objective. For low-power objectives cover glass thickness may not be an issue, but the loss of Working Distance maybe enough to prevent focus at
Infinity, since the lens may hit the cover glass before focus at Infinity is reached!
Immersion objectives are a bit different and are covered next...
The basic effects of the Finite > Infinity transition are summarized in the chart on the following page.
The upper section covers “Dry” lenses and the middle section covers Immersion types. The lower section covers Notes for both types. Note that Oil and Water immersion types are considered differently. Finally, this Table applies to 160mm and 170mm T.L. finite objectives only.
-1-

2. Immersion Objectives > Infinity
Immersion objectives react slightly differently to a Finite > Infinity shift. Also, there are two different types of Immersion lenses of concern here, Oil immersion and Water immersion. For Oil immersion types, the optical characteristics of the immersion fluid is very similar to that of a cover glass, so the presence or absence of a cover glass makes little difference ( other than, perhaps, limiting focus movement ).
This is not the case for Water immersion, however, especially at higher NA's. For these lenses a "correction collar" is often built-in to allow adjustment for cover glass thickness variations from nominal (e.g: 0.17mm).
So, in a way, these lenses can behave somewhat like "dry" lenses when they are shifted to Infinity use.
Now, Finite Immersion objectives normally fall into a rather limited range of Focal Length (Magnification) ranging from about 4mm (~40x) down to less than 2mm (100x), which means there is much less variation in their response to Infinity shifting. In fact. many of the 100x lenses exhibit such minor changes as to be useable at Infinity as if there were no changes at all! ( This is particularly true of Oil immersion types with a Focal Length of < 2mm and a NA of <1.30 — essentially, your basic 100x Oil immersion types.
)
However, it is at the other end of this range, the ~4mm (40x-60x) types where things can get interesting!
One advantage inherent in immersion-type lenses is the freedom to alter the immersion fluid. Although this might at first seem trivial, it is in fact a very distinct advantage. With "dry" lenses a shift to Infinity focus results in a need to use a thinner-than-normal cover glass , if image quality is to be preserved. However, with
Oil lenses, there is no such "easy fix" available. But, instead of using a different cover glass thickness, the optical nature of the object space of an immersion lens may be altered simply (and more easily) by changing the characteristics of the Immersion fluid!
This is not as critical with 100x lenses since the changes there are usually small and of relatively little consequence, especially if these lenses are used in a "routine" manner (e.g: with a 0.9 NA Condenser or an
“Oil” type used without oil.) Unfortunately, this is not so much the case with medium-power (40x-60x) immersion types, which are more greatly affected by a shift to Infinity. With these, the image quality can suffer at NA's much above 0.75-0.80, or so, if they are used with normal immersion oil.
What happens with these objectives is that they undergo the same type of effects as high-NA dry lenses, including “cover glass thickness,” regardless of whether or not a cover glass is actually present ! But, while this may at first seem an insurmountable problem, it actually creates a whole new set of opportunities…
Welcome to the World of “Multi-Immersion”, courtesy of Infinity Shifting!
If we abandon Conventional Thinking, for a moment, we can quickly see that the answer to this problem with medium-power immersion objectives is simply to choose a different immersion fluid . And, for the range of lenses considered here, the choices are quite simple: Immersion Oil (RI=1.52), Glycerin ( RI=1.48
),
Glycerin:Water, 1:1 ( RI=1.40
), and Water ( RI=1.33
).
Given these choices, the answer to the image quality problem then becomes simply a matter of choosing the immersion fluid which gives the best image quality for the lens in use. And, even more simply, choosing the best fluid from an even shorter list, based on the Magnification of the objective:
Magnification: 40x-50x*
Imm. Fluid(s): Water
Glyc.:Water
50x-63x
Glyc.:Water
Glycerin
75x-100x
Glycerin
Imm. Oil
* See discussion on Water Immersion use, on following page.
-2-

Now, as with dry lenses, the sensitivity of the lens to these effects is determined by its Effective NA -- the resulting NA of the lens when the Condenser Iris is brought into use. This creates another, and perhaps even simpler option, just use the lens normally (e.g: with its normal immersion fluid) and adjust the Effective NA of the system with the Condenser Iris until a satisfactory image is achieved -- a method which can work quite well for routine work.
( Note that for Darkfield work, an Iris in the Objective is almost mandatory.
)
However, in taking this simple "short cut" we can be missing a great opportunity.
It just so happens that finite-type Oil immersion objectives in the 40x-50x range often perform very well at
Infinity when Water is used as the immersion fluid. When this is the case, it results in an opportunity to produce a good-performing Water Immersion objective, simply by using an appropriate finite-type Oil immersion lens on an Infinity scope! ( Note that, for decent performance, of course, all traces of oil should be removed from the lens before its immersion in any water!
)
Naturally, it would be nice to have some simple, quantitative means to test these various alternatives, and there most certainly is one -- just use the Condenser Iris, and note its opening when a satisfactory image is obtained. If the lens performs acceptably when the Iris is fully open, then the immersion fluid in use is satisfactory. Just be aware that for lenses of moderate NA (<0.85, or so) there may be little difference detected, as these types are less sensitive to the choice of fluid. However, for higher NA's (e.g: 0.90 or more) the choice of fluid is usually much more significant. ( This is more true for lenses in the 40x-60x range than for those of higher Magnification.
)
And, as always, the Condenser Iris may be used to reduce the lens' sensitivity with any of these fluids -- all that really matter is achieving a satisfactory image!
3. System Issues > Infinity
One factor in the 'Finite > Infinity Shift' process not yet discussed is the Infinity Tube Lens and its effects.
In classic 'Finite' systems there is only the Objective and Eyepiece(s) to consider. However, with Infinity systems there is a third component, the so-called "Tube Lens," which is inserted between the two. The existence of this nefarious device can often have significant and unexpected effects on the final image. So, it needs to be properly considered in terms of the overall system, if success is to be achieved.
Normally, it is undesirable to have to constantly switch eyepieces when changing objectives, although with
Finite > Infinity switching this is sometimes unavoidable. Still, with proper choice of optics this task may often be minimized, if not eliminated altogether!
Just as with eyepieces, a Tube Lens may have many characteristics, color compensation being just one of them. Fortunately, with many modern systems the trend is to provide such compensation within the objectives, leaving the Tube Lens as mostly a "passive" element -- providing a "focus shift" for the eyepieces and little else. Such systems are the easiest to use for Finite > Infinity shifting. Many Japanese systems
(Nikon, Olympus, Meiji, etc. seem to be of this type.
Other systems can deliver a fully-corrected image without any eyepiece at all, relying on the Tube Lens to provide any necessary final image corrections. These systems may or may not be as easy to use, depending upon exactly what the Tube Lens functions really are. European designers seem to favor this approach (e.g:
Zeiss, Leica, Reichert, Wild-Leitz, aus Jena, etc.).
( Note: For more on this, see Sections 5., Other System Issues .)
-3-

With any of these systems, the difficulties arise when any "foreign" optic (Finite or Infinity) is used. In the case of Infinity objectives from a different system, image problems can occur when the requirements of the new lens differ from those of the "normal" objectives for that system. German systems reportedly include additional corrections into their Tube Lenses, such that (and perhaps even intentionally) Japanese objectives may suffer performance impairments. In other words, German - German and Japanese - Japanese "Infinity" pairings may give the best results.
(Note here that the Reichert/USA and Leica/USA Infinity offerings
[RMS-DIN mounts] seem to belong in the Japanese column.)
Unfortunately, things are not so neat and tidy when it comes to Finite > Infinity pairings. Since all Finite objectives were designed, inherently, for use without any Tube Lens, the best results may be expected when they are mounted on Infinity systems in which the Tube Lens has no, or few, added correction functions. In such cases, where only additional color- correction is required for the Finite objectives, this may often be obtained simply by using the original Finite-system eyepieces, or an equivalent. In cases where the Tube
Lens provides some color compensation, Finite-system eyepieces of milder compensation may perform better.
Another factor inherent with most Infinity systems is unanticipated Magnification. Because the projection distance of the image to the eyepiece is typically much longer in Infinity systems than finite systems, the final image magnification may be considerably increased with the Finite > Infinity shift. Magnification factors for a number of common Infinity systems are listed in Table 2, following the next Section.
4. Special Cases
Up to this point we have covered only normal objectives; those having 'fixed' cover glass correction (e.g: typically, 0.17mm). However there is a whole class of objectives, with relatively high NA's, which provide simple means for the user to adjust the correction as needed, although usually over a somewhat limited range.
This sort of begs the question, "But what about using one of these for Finite > Infinity shifting?"
Well, while this, at first, might sound like an ideal solution, there are (of course) a couple of issues to consider. First, the problem of shrinking Working Distance remains, unchanged. Next, the problem of
Reduced Cover Glass Thickness does not simply go away either. Instead, what happens is that whatever
Cover Glass Thickness change develops, due to the Finite > Infinity Shift, now affects the Cover Glass
Adjustment mechanism of the lens.
For example, given a lens with a normal (Finite) adjustment range of, say, 0.11 to 0.22mm, and a C.G.
Thickness change of 0.10mm, the effective adjustment range of the lens would become 0.01 to 0.12mm. So, in effect, when used at Infinity, this lens would become a "No Cover Glass" (NCG) type, unless you just happen to have a lot of very thin cover glasses!
This can become more interesting, however, when Water immersion lenses are considered. Many of these having higher-NA (e.g: 0.80 or more) rely on a correction collar to adjust for variations in cover glass thickness. However, when used at Infinity, this collar can often be set to allow the lens to be used without any cover glass – e.g: for Direct immersion (“dipping”) use!
However, that is not the end of the story. Note that although the adjustment range get very close to zero thickness (NCG), the tolerance for this setting can be very tight. In other words, because the lens can quite accurately be set to take advantage of its new environment, it most likely can be used at its full rated NA!
And, other examples exist for lower-power objectives…
-4-

Consider, for example, a 40x/0.55 LWD objective -- a common lens for inverted microscopes. These usually have an adjustment collar for a wide range of cover glass thicknesses, 0.0-2.0mm being fairly common. With a lens like this, the C.G. Thickness shift associated with Finite > Infinity shift is a mere 0.2mm, making the range at Infinity -0.2 to 1.8mm. Similarly, the Working Distance Change of 0.1mm is of little practical consequence.
Therefore, for lenses like this, the shift to Infinity use has little practical impact on the objective's performance. And, for the 20x versions of these objectives, the results are similar.
One subject, mentioned earlier and not yet discussed, is the issue of low-power objectives. As shown in
Table 1, these can exhibit a significant loss of Working Distance when shifted to Infinity focus. And this is especially true of many rather desirable Plan types (e.g: Zeiss Planachromats) which may not even be able to achieve focus when used at Infinity.
This raises two corollary issues: (1) most low-power Infinity objectives don’t have a lot of Working
Distance, and, (2) there typically aren’t a lot of choices for Magnification (between 2x and 10x) when it comes to new Infinity lenses.
Olympus has a very nice 4x Plan for their UIS/UIS-2 system, with a W.D.=22mm. They also have a similar
5x M-Plan with W.D.=20mm, but these lenses are exceptions to the general offerings. And, although they feature RMS mounts, they may not integrate well with other Infinity systems, particularly those which have added corrections incorporated into their Tube Lens.
So, what to do if you want a low-power objective with a decent Working Distance for use on your new
Infinity scope? One option is to “turn back the clock” and use one of the older, non-plan finite objectives!
Of these, only a few common types are likely to be truly useful, as listed below:
Mfgr. and Description Mag @ Inf. Parf. Dist.* W,D, @ Inf.
AO 4x/0.10 (Cat. no. 103) 4.8x
AO 6.5x/0.2 (Cat. no.3006) 6.6x
AO 5x/0.14 (Cat. no. 107) 7.5x
AO 5x/0.20 (Cat. no. c107) 7.2x
34.0mm 16.5mm
34.0mm 17.5mm
28mm
29mm
12mm
15mm
B&L 4x/0.1 (32mm, 0.10)
B&L 2.6x (40mm, 0.08)
B&L 2x (48mm, 0.08)
B&L 10x Divisible obj.:
Olympus M5x/.010 [3]
5.6x
4.6x
3.8x
4x (Rear only, older) [1] 6.3x
4x (Rear only, newer)[2] 6.3x
6.0x
44.5mm 31mm
39.5mm 25.5mm
46.5mm 33.8mm
40.0mm
40.0mm
35.0mm
25.3mm
21.3mm
20.7mm
Notes:
[1] Older version, 2 rounded knurled rings on chrome barrel.
[2] Newer version, 1 flat knurled ring on chrome barrel.
[3] Later version for BH-M, all-black finish, no chrome.
* Parfocal Distance is listed for Infinity focus and some systems
may require an extension or adapter. All lenses above are RMS .
-5-

5. Other System Issues
5.1 Tube Length Issues
Thus far, the discussions have centered around “standard” (e.g.: 160mm or 170mm Tube length) lenses.
However, this would seem to beg the question, “But what about using longer T.L. objectives, like 210mm or 250mm?”
The answer to this may surprise you!
While, at first glance, it might seem that this path would be somehow “easier” (after all, 250mm sure seems closer to Infinity than 160mm does), this quickly proves not to be the case. And, instead of being better-off, you can easily wind up being worse off. Here’s why…
Objectives for longer focal length systems require a longer focal length to achieve the same final image
Magnification. For example, to achieve 10x, a 160mm objective only needs a focal length of 16mm, but to get 10x in a 250mm finite system requires a focal length of 25mm. Consequently, the 250mm objective suffers a greater reduction in Working Distance ( more than 4mm, versus 1.8mm for a 16mm lens ). Further, the Reduction of Cover Glass Thickness will be 2mm or more at NA 0.25, so the useable NA of the lens maybe reduced as well. Finally, nearly all the ‘longer-than-standard’ tube length finite objectives were designed for use in Metallurgical systems – that means with No Cover Glass (NCG). And this only makes matters worse if you work primarily with covered specimens!
Before you go running off to your favorite Internet auction site to stock up on old Reichert
(250mm) objectives, I suggest you give the above paragraph some thought…
So, next you ask, “But what about Intermediate T.L. systems, like 210mm (Nikon CF) and 215mm
(Olympus BH, etc.)?”
Objectives for these are not as severely handicapped as those for 240mm and 250mm systems, but many of the same concerns still apply (just to a lesser degree). Most of these are useable on existing systems, and mostly in the Industrial market, where both choices and supply may be more limited, and where prices are often much higher. So, while you may be able to find the lens you want/need, you may also find that it is simply too costly, compared to your alternatives – especially if it says “Nikon” on the barrel.
Also, many (if not most) of these Industrial-type objectives use non-RMS mounts and/or incorporate special features which increase the cost, but are of little use with covered ( Biological ) specimens (e.g: BF/DF epiillumination capability ).
.
If for some reason you actually do require a NCG objective, I suggest you read the first part of Section 4 for some tips on possible alternative choices, based on “standard” objectives.
-6-

5.2 Magnification Issues
This factor seems to be largely overlooked by average users, although it can play an important role in the determination of final image quality. Seasoned users may recall the “Empty Magnification Rule,” namely, that the final magnification (for visual use) “should not exceed 1000 times the Objective NA.”
Ordinarily, this is not an issue, but it arises here fir two separate reasons: (1) the Effective NA is often much less than the Objective NA, if and when the Condenser Iris is closed somewhat to improve image contrast, and, (2) because the Magnification Factor, due to the Tube Lens in the Infinity system, has not been taken into account.
To appreciate this better, just consider a simple case: a 40x/ 0.65 (4mm) Finite objective used on an Infinity system having an f=200mm tube lens – both are common equipment. If we adjust the Condenser Iris to secure the best image contrast, we fine that the Effective NA of the system is perhaps 0.50, or so. Not all that bad – and usually adequate for a 10x eyepiece.
However, when we consider the Magnification Factor due to the Tube Lens (e.g: 1.25x) we now find that this setup just barely satisfies the Rule. Now, if we try to use a 12.5x eyepiece we will find the image degraded – not due to any fault of the objective but merely due to the user overlooking the Tube Lens’
Magnification effect!
5.3 Color Compensation Issues
Many users get all excited about this issue, as if it was the primary determinant of Image Quality…
I can assure you that it is not!
In fact, it has little to do with the more important image factors, like image contrast and sharpness. Instead, it deals mostly with color-fringe artifacts in strongly-colored and/or high contrast images. But, even more significantly, it becomes most visible (and potentially objectionable) only toward the edges of the eyepiece field.
In Finite systems color compensation is managed by dividing the function between the objective and the eyepiece, according to the manufacturer’s design plan. However, the current trend, enabled by new optical glass technology, is toward accomplishing the function entirely within the design of the objective.
Now, as this is the practice on many (if not most) newer Infinity systems, it raises the issue of just how to deal with the color-compensation requirements of Finite objectives when used on these systems.
The general approach is one of trial-and-error, usually beginning with whatever eyepiece(s) the subject objective functioned best with in its normal, Finite, use. However, there may be complicating technical issues, such as compensation incorporated into the Infinity Tube Lens, or issues of Magnification factor, which may require selecting an eyepiece with less compensation.
In general, if the image shows Orange fringes on outside edges, especially near the perimeter of the image, then the image is Under-corrected, and an eyepiece with more compensation is needed . On the other hand, if the image shows Orange fringes on the inside edges, then the image is Over-compensated and an eyepiece with less compensation is needed .
As a rough guide, eyepiece compensation may be estimated by examining the residual color fringe around the Inside edge of the Eyepiece Field, when viewed against a white background. If it is of a yellow-orange color, the eyepiece is a compensating type – and the more intense the color, the greater the compensation.
( However, be aware that some modern compensating eyepieces, by design, will show no such colors.
)
-7-

Manufacturer
AO (5 0 & 60)
Zeiss (IC S)
Zeiss (IC S)
Meiji
Olym pus (U IS)
Olympus (LB)
AO (s tanda rd)
Leica (W ild)
Reichert (USA)
Leica (Delta)
Meiji
Nikon
Nikon (CFI-60)
Nikon (CFI-60)
Leitz
Zeiss (A xiom at)
Zeiss (aus Je na)
( Revised: 29 Sept 2016 )
Tube Lens Mag.Chg.
Pa r. Dist.
Thread Remarks
163mm
165mm
165mm
180mm
180mm
180mm
183mm
183mm
183mm
200mm
200mm
200mm
200mm
200mm
250mm
250mm
250mm
1.02x
1.03x
1.03x
1.13x
1.13x
1.13x
1.14x
1.14x
1.14x
1.25x
1.25x
1.25x
1.25x
1.25x
1.56x
1.56x
1.56x
34mm
45mm
45mm
45mm
45mm
45mm
34mm
45mm
45mm
45mm
60mm
45mm
60mm
60mm
45mm
45mm
45m m `
RMS [ Obso lete ]
RMS Bio.
Var.
Met.
RMS Bio.
RMS Bio.
M26 Met.
RMS Bio.
RMS Bio.
RMS Bio.
M25 Bio.
M26 Met.
RMS Met.*
M25 Bio.
Var. Met.
RMS [ Obso lete
M27 Bio.
Var.
Met.
]
Notes : Bio = Biological objectives, typ. 0.17mm C.G. Met = Metallurgical, typ. No Cover Glass (NCG) .
* Interim system -- Limited availability new.
Var. = Various mountings, depending on use.
Mag. Chg. reflects change for 160/170mm T.L finite objectives mounted on the listed system.
Table 2 – Infinity Optical Systems Summary