Subjective refraction
OP1201 – Basic Clinical Techniques
Spherical refractive error
Dr Kirsten Hamilton-Maxwell
Today’s goals
 By the end of today’s lecture, you should be able to
 Explain the different methods used to determine the spherical
component of your patient’s refractive error
 Explain the concept of Best Vision Sphere (BVS) and use several
methods to determine it
 Describe the advantages and limitations of the different tests in
different situations
 By the end of the related practical, you should be able to
 Refine the spherical component of your retinoscopy result
 Determine the BVS of your patients using a variety of techniques
from scratch
 Complete the task within 10min for both eyes
Background
What is subjective refraction?
Types of spherical refraction
General set up
Subjective refraction
 Aim is correct your patient’s refractive error by
asking them choose what they like
 Guided by you, of course
 It is a multi-step procedure that we will cover
over the next few weeks
 First step is to check that the spherical correction
is correct following retinoscopy
 We will start spherical refraction today
 First, let’s see how it all fits together
Subjective refraction
RE
LE
Check sphere
Check sphere
Check cyl
(Axis and power)
Check cyl
(Axis and power)
Recheck sphere
Recheck sphere
BE
Binocular balance
Final prescription (Rx)
Today’s
topic
Spherical refraction
 There are a number of different procedures
 Plus/minus test
 +1.00DS test
 Duochrome test
 All are monocular
 Rule of thumb – “give the most positive lens that
provides maximum vision”
 Minimise accommodation = increase comfort
 There is a twist to this rule when preparing for cross-
cyl
Set up
 Is the same for all of these procedures
 Assume that retinoscopy has been completed, the
working distance lens has been removed and the
vision measured
 (For now, assume that the cylindrical correction is
correct)
 Turn the room lighting back on
 Occlude the eye that is not being tested
Plus/Minus Test
Effect of plus and minus
Positive lens
Emmetropia
Both lenses equally clear
(Blur circles same size)
Negative lens
Plus and minus in myopia
Positive lens
Uncorrected myopia
Negative lens is clearer
(Blur circles different sizes)
Negative lens
Plus and minus in hypermetropia
Positive lens
Uncorrected hypermetropia
Positive lens is clearer
(Blur circles different sizes)
Negative lens
What does the patient see?
Clear
Blurred
Blurred
Too much plus
Too much minus
-0.50DS
0
-0.25DS Same/smaller and darker
-0.50DS Smaller and darker/blurred
Vision influenced by accommodation
+0.50DS
+0.25DS Not as sharp
+0.50DS Fainter/blurred
You are expecting a vision change of one line per 0.25DS!
Plus/minus technique

This information can be used to determine the
refractive error using the plus/minus technique
Add +0.25DS and ask “are the letters clearer,
more blurred or the same?”
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Use the letter chart as a target
Ask your patient to look at one line bigger than
current vision
Use a larger lens power if vision poor (eg. +0.50DS
or +1.00DS)
Plus/minus technique

If vision improves or remains the same,
exchange the current spherical lens in the trial
frame for one that is +0.25DS higher


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If you used +0.50DS or +1.00DS, then change it by
+0.50DS or +1.00DS respectively
If you are testing a hypermetrope with
accommodation, do not remove the trial frame lens
until the new one is in place
Repeat until the vision begins to blur

When it blurs, it is time to stop adding plus
Plus/minus technique
Now check with a -0.25DS lens and ask “are the letters
clearer, more blurred or the same?”
If there is an improvement in vision, incorporate 0.25DS into the trial frame.
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If there is no improvement in vision, do not change the lens
power!
This means that they must read more letters on the chart!
If the patient reports their vision is better but they
cannot read more letters, ask “do the letters definitely
look clearer, or is it just smaller and darker?”


If clearer, add the lens
If smaller and darker, do not add the lens!
That last point again!
 Check that changing lens power changes the
acuity as well
 Expect a change of 1 line per 0.25DS
Never incorporate extra minus into your
prescription unless you can measure an
improvement in vision on the letter chart!
How to ask the question!
 What you say is important as you can lead the
patient towards a particular answer without
realising it
 You’ll get the WRONG answer if you are not careful
 When adding plus…
 Is it better, worse or still the same?
 Compare that to “is it better?” only
 You could reject a lens that was “the same” when it is actually
telling you that your patient had been accommodating to make
their vision clear
 When adding minus…
 Is it clearer, or smaller and darker?
Plus/minus technique
In summary…
You are aiming to give the patient as much plus (or
as little minus) as possible so the lens you want is
the one in between…
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Where adding more plus would cause blur
Where adding minus does not improve vision
“They look the same”
 If patient answers “Same” or “No difference” at
first presentation
 You already have the right lens
 Confirm by changing Rx power to demonstrate a
definite response
 Vision is too poor due to inaccurate retinoscopy
result, or pathology is present
 Increase lens power and repeat
 Small pupils
 Increase power and repeat
 Be wary of inducing accommodation!
+1.00DS Blur Test
+1.00DS blur test
+1.00DS
 Over-plussing should induce a blur circle on the retina,
hence reducing vision
 Should blur by 1 Snellen line per 0.25DS
 If distant light currently focussed on retina, +1.00DS
should cause 4 lines of blur
 Should blur back from 6/6 to 6/18
Performing the +1.00DS blur test
 Insert +1.00DS and ask patient to read down the
chart
 If not 6/18, adjust spherical power
 Remember 1 line per 0.25DS
 Repeat until vision is 6/18
 Remove +1.00DS and check that vision is no
worse
 If it has, you have added too much plus!
 Let’s look at the optics
+1.00DS – Vision better than 6/18
+1.00DS
 If vision with +1.00DS is better than 6/18, then blur circle must be
smaller than expected
 Explained by focal point behind the retina - you have not added
enough plus, or you have added too much minus
 Action
 Reduce minus or add plus
 By 0.25DS per line better than 6/18
+1.00DS – Vision worse than 6/18
+1.00DS
 If V/A with +1.00DS is worse than 6/18, then blur circle must be
bigger
 Explained by focal point in front of retina, so you have added too
much plus or not enough minus
 Action
 Add minus or reduce plus
 By 0.25DS per line worse than 6/18
Be aware!
 British standard Snellen chart is missing the
6/7.5 and 6/15 lines of the LogMAR chart, so 4
lines of blur can sometimes appear to be only 3
 Pupil size is important
 Reduced pupil size can also reduce the size of the blur
circle
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For example, an emmetrope may only be blurred by 2 lines
despite 1.00DS of uncorrected refractive error
If you added plus to blur the extra two lines, you will
overplus!
Be wary is elderly patients with small pupils
 Large pupil has opposite effect; will blur back too
quickly
Be aware!
 Not everyone starts from 6/6
 With greater amounts of blur, 0.25DS per line
relationship breaks down and becomes less
accurate
 Vision will generally underestimate spherical error
 Results may be unusual if ocular pathology
 If change in power is significant (e.g >0.50DS),
perform +1.00DS again to double check results
and confirm with alternatives
 When in doubt, use another test to confirm
Duochrome Test
Duochrome test
 Uses longitudinal chromatic aberration to determine the refractive
error
 Whichever colour is focussed nearest to the retina will be seen as
clearest
 Emmetrope = equal
 Myope = red clearer
 Hypermetrope = green clearer
Optical principles of duochrome test
Chromatic aberration
Prismatic effect of lens
leads to dispersion
Optical principles of duochrome test
0.50DS
Performing the duochrome test
 Switch on duochrome test and establish that the
patient can see the ring targets
 Ask “Are the circles sharpest and clearest on the
red or on the green background?”
 Alter power by 0.25DS according to patient’s
response
 Minus if red clearest, plus if green clearest
 Repeat until no difference seen
 Be wary of accommodation and red-preference
Limitations of duochrome
 The ring targets are usually constructed of ring
thicknesses equivalent to 6/9 (inner) and 6/12 (outer)
Snellen equivalent targets
 Will not work if vision is less than 6/12
 The difference in focal position due to chromatic
aberration is 0.50DS
 Will not work if prescription is significantly incorrect
 Small pupil will reduce size of blur circles
 Difference between the clarity of red and green is reduced
 Reduce room lighting for older patients
 Always be aware of the alternative tests!
“They look the same”
 If patient answers “Same” or “No difference” at first
presentation
 Duochrome is balanced
 Confirm by using +0.25DS (red now clearest)
 Rx too far out
 Use other tests, when vision 6/12 or better, return to duochrome
 Small pupils
 If no change in response with lens change, move to another test
(though the effectively of all is reduced)
 Vision too poor due to pathology
 Abandon duochrome – try plus/minus test with large steps instead
Limitation?
 What if your patient is one of the 8% of the
population that have a red/green colour vision
deficiency?
 It still works! Why?
 Refer to top/bottom of the chart instead of red/green
My ret was a disaster
Now what?
What if my ret result is a disaster?
 If vision is poor after retinoscopy, or you don’t have a
retinoscopy result…
 Don’t panic - think about what you already know
 What did your patient tell you?
 Vision – distance vs. near blur, pinhole
 (Current correction)
 Check sphere power
 You will need to find the best vision sphere (BVS)
Best vision sphere
 Best vision sphere is literally the lens that gives the best
vision with a sphere only!
 Use the plus/minus test described above, but using larger
steps (±0.50DS or greater)
 Provides
 Crystal clear vision for simple myopes and hypermetropes
 The best possible vision for an astigmat because the circle
of least confusion will be on the retina; the remaining blur is
due to the cyl alone
 Record your result and vision, then check for astigmatism
(as described next week)
For a myope
“My vision is blurred”
“That looks great!”
Simple myopic astigmatism
Circle of Least Confusion
Focal lines are equally blurred
“It’s very blurred”
Blur is due to combination of…
CLC in front of the retina
Focal lines being separated
Interval of Sturm
Distance between the focal lines
With BVS
Circle of Least Confusion
Has moved, is now on the retina
Interval of Sturm
“That’s better but it still isn’t clear”
Length unchanged
Reason the vision is still blurred
All blur is now due to uncorrected cyl: We will learn how fix that next week
BVS
BVS = Sphere + ½cyl
The spherical equivalent is calculated the same way
Routine
Suggested routine
 This will depend on your patient, but a suitable
routine could be…
1. Retinoscopy
2. +1.00DS blur test
3. Plus/minus test
4. Duochrome (to confirm that you have found the
correct sphere power)
By the end…
 You have given the patient the best acuity that
you can
 You have given the patient the most positive lens
that gives them this vision
 You have checked that the addition of +0.25DS makes
vision worse
 You have checked that the addition of -0.25DS does
not make vision better
 You have written your result down and recorded
the vision for each eye
Recording results
 There is a box for RE and LE
 You only “need” to record the final result
 BUT you may find it helpful to write down your
results and vision from the individual tests while you
are learning
Things that can go wrong
Common errors
 Forgetting that these tests are monocular
 Not monitoring vision as you go
 Which can result in adding too much minus or plus
 Using poor patient instructions
 Assuming that 6/6 is the endpoint
 Not listening to the patient and/or listening too
much to the patient
 Not remembering that this is difficult for your
patient
Further reading
Read Elliott, Section 4.9-4.12
Review Elliott Online