Introduction to Single Lens Reflex Cameras
Part 6: The Aperture Sharpens the Picture
Chapter outline
1. What is aperture?
1.1. Aperture mechanism
1.2. What is an f-number?
2. What changes with aperture?
2.1. Aperture and brightness
2.2. Depth of field and blur
2.3. Unique lenses to control blur
3. Techniques using aperture
3.1. Maximum aperture with a telephoto lens
3.2. Minimum aperture with a wideangle lens Multiple exposure
3.3. Changing blur with a handmade aperture
1. What is aperture?
In the human eye, the aperture is referred to as the iris. In the center of the iris is a
circular opening called the pupil. The diameter of the pupil contracts in bright
environments and expands in dark ones, adjusting the amount of light that reaches the
retina.
Eye color depends on the color of the iris. If the surface of the iris has a low pigment
density, eyes are blue. If pigment density is high, eyes will be brown or black.
The diameter of the human iris is a concentric circle, so the pupil is always a circle. A
cat's iris is a vertical slit and a horse's is a horizontal oval. Heart-, diamond- and gourdshaped irises also occur in nature.
1.1. Aperture mechanism
The size of an SLR camera's aperture is controlled by the movement of several aperture
blades that behave much like the human eye, changing the size of the circumference. This
type of aperture is known as an "iris aperture". For artistic blur, the aperture should be as
circular as possible. To achieve this, there should be an odd number of shutter blades in
the shape of a polygon.
When you look through the viewfinder of a modern SLR camera, the lens aperture is
always fully opened so the image is bright and easy to put into focus. When the shutter
release button is pressed, the aperture blade is adjusted to the aperture value that has been
set. At the same time, the reflex mirror in the camera body flips up and the shutter opens.
When the shutter closes, the mirror moves back to its original position and the aperture
returns to a fully opened configuration. Despite the speed of this operation, it is quite
complicated. Sometimes it appears that the image in the viewfinder is different than the
actual picture. Aperture setting is one reason for this. Because the image seen in the
finder is observed with the aperture fully opened and the actual photo is taken after the
aperture has been adjusted, there is a disparity.
Some SLR cameras have a stop-down (preview) mechanism that enables you to see what
the actual picture will look like. Though this image may appear a bit dark, it is an
accurate image representation and a good way to check depth of field. (The finder image
might appear a bit more focused than the resulting photo, depending on the quality of the
finder screen's matte surface).
A lens' aperture blade (Ai AF Nikkor 50mm/f1.8)
Fig 1.a.
An aperture ring is included with
this lens.
Depending on manufacturer, you
can see the aperture blades of the
lens moving when you rotate the
aperture ring.
The aperture is set at f2.
Fig 1.b.
The aperture is set at f8
Fig 1.c.
The aperture is set at f22
1.2. What is an f-number?
Aperture size is indicated by an "f-number." Many cameras note f-number as ...1, 1.4, 2,
2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64, 90...... Though the arrangement of these numbers
may seem strange notice that every other number is doubled. Each step is a multiple of
the square root of two. These numbers indicate the lens' focal length (f) divided by the
diameter of the aperture (d). Expressed as a formula, that is "F = f / d".
The amount of light that penetrates the lens, the image brightness, is proportional to the
size of the aperture. Therefore, it is inversely proportional to the square of the f-number.
The amount of light is halved with each increasing f-number increment. However, this is
the case only when the subject is on the optical axis and is relatively far from the camera,
or in other words, when the shooting magnification is relatively small. If "m" equals
shooting magnification, then image brightness is inversely proportional to the square of F
(1+m). So, when taking close-ups using an extension ring or bellows, a positive ("+")
compensation is necessary. This adjusted f-number is called the effective f-number.
(NOTE: If your camera has internal TTL metering, it is not necessary to make this
calculation).
Also, with some inexpensive wideangle lenses that are used with compact cameras and
some expensive wideangle interchangeable lenses for rangefinder cameras, the decrease
in light amount around the frame of the picture (= the four corners of the picture appear
dark) becomes obvious.
When the aperture is fully opened, the f-number is referred to as maximum aperture or
maximum f-number. These f-numbers are usually shown on the camera (1:1.4, F1.4,
f/1.4, etc.). Conversely, the smallest aperture is called the minimum aperture.
By using a lens with a small maximum f-number, you will be able to take brighter
pictures. Of course, this is advantageous when shooting in dark places or when you want
to blur the background. In order to reduce the f-number, the diameter of the lens must be
large, because it must be proportional to the focal length. This poses several practical
problems, namely that the lens becomes bigger, heavier and more unwieldy and more
expensive. Lenses of this type are often called bright lenses or high speed, though these
names don't actually denote high transmittance. In theory, the maximum brightness of a
lens is f / 0.5. Though it seems logical that a lens with a larger diameter would make an
image brighter, this just isn't the case.
2. What changes with aperture?
We now know that brightness is affected by aperture size, as is depth of field. This
information always appears in an SLR's manual, so it should be clear to the user.
Though it's easy to change the aperture size by adjusting the lens, it is often difficult to
get the desired results, depending on the purpose and subject of the photograph. The first
step to familiarize yourself with aperture is to learn how to set it yourself using manual
mode or aperture-priority AE mode ("A" mode). Another good idea is to make large
changes in aperture size: One or two steps of aperture change might be enough to realize
notice able differences in your photos.
As standard procedure, try taking three pictures with the following settings, and compare
the results:
1.) maximum aperture,
2.) minimum aperture and
3.) f-number set somewhere in between.
2.1. Aperture and brightness
Figure 2.a. to 2.c. were taken with manual exposure with the same shutter speed. Only the
aperture was changed. Figure 2.b. is at normal exposure, f5.6. Figure 2.a., at f2, is three
(3) steps overexposed. Figure 2.c., at f16, is three (3) steps underexposed. The difference
is apparent when the aperture is set at +/- 3 EV, for a total margin of 6 EV. This does not
mean the image will not be visible; if these photos are regarded as portraits, then figure
2.a., three steps overexposed, might be considered the best. This helps highlight the value
of taking a series of photos with different aperture values, then comparing them.
Exposure difference due to aperture setting (using Ai AF DC Nikkor 135mm f/2 lens)
Fig 2.a.
f2 (3 steps over-exposed)
Fig 2.b.
f5.6
Fig 2.c.
f16 (3 steps under-exposed)
2.2. Depth of field and blur
Figure 3.a. to 3.c. was taken using the same aperture values as figure 2.a. to 2.c., only the
values have been set at aperture-priority AE mode and different shutter speeds to achieve
the appropriate exposure.
Background blur in each photo differs greatly. With an open aperture (3.a.), the
background is significantly blurred, but with a close aperture (3.c.) there is only slight
blurring. Adjusting the aperture allows you to make a difference in the distance-focus
relationship. The amount of blur is generally proportional to the diameter (d) of the
aperture, and is inversely proportional to the f-number (F). Blur is proportional to the
lens' focal length (f). Also, the wider the focusing plane and the wider the portion of the
image that appears in focus, the more blur there will be.
Depth of field is the area in which the image appears to be in focus. This means that the
smaller the aperture is, the deeper the depth of field will become. So, depth of field
changes with aperture, the lens' focal length and shooting distance. The general rules are
shown below.
But be cautious when shooting with telephoto lenses (long focal length) or when shooting
close-ups (short focusing distance), because depth of field becomes quite shallow.
Depth of field
Focal length
Shooting
distance
Deep
Short
(Wideangle)
Long (Far)
(Large aperture
value)
Shallow
Long
(Telephoto)
Short (Near)
(Small aperture
value)
Aperture
Small
Large
Changes in depth of field due to aperture (DC Nikkor 135mmf/2D)
Fig 3.a.
f2
(Notice the background !)
Fig 3.b.
Fig 3.c.
f5.6
f16
2.3. Unique lenses to control blur
There are interesting lenses available today that can be used to help control blur. Though
this is not directly related to the subject of aperture, it is worth our while to examine some
of these lenses.
The AI AF DC Nikkor 105mm f/2D and AI AF DC Nikkor 135mm f/2D lenses have a
"defocus control" (DC) function that can control the amount of blur that appears in a
photo. They can cause "desirable blur," in which the point source blurs in a circle, leaving
good resolution in the center of the blurred image. There will be a slight haze in the
center of the photo, so the frame gradually fades and does not show the line.
In addition to the aperture ring, on the barrels of these lenses is a DC ring. By setting this
ring to "F" (front), the front portion of the subject will become blurred; by setting it to
"R" (rear), the rear part will blur. On the DC ring is a number similar to the f-number. By
setting this number larger than the shooting aperture, the subject will become softly
focused. (NOTE: Check the focus each time the DC ring is adjusted).
You will not be able to notice a major difference when looking through the viewfinder,
even when the DC ring has been adjusted. But in the final outcome there will be a marked
difference. These lenses are quite special, and are especially suited for portraits.
Defocusing function (Ai AF DC Nikkor 135mm f/2D)
Fig 4.a.
Front-blur priority mode : The DC ring is set to "F" (taken at maximum aperture, f2)
When an appropriate spherical aberration has been set, the light in the foreground will collect in the center
of the subject and a haze will surround it.
The entire photo will have a smooth blur. Light is concentrated on the foreground of the focusing plane
and gradually fades outward from the center. Background light, however, blurs in a spherical fashion.
Fig 4.b.
Normal mode : The DC ring is set to "0" (Aperture : f2)
Because the spherical aberration is almost completely compensated for, light energy will be concentrated
in two spots, or "caustics." Both the foreground and the background will be blurred.
Fig 4.c.
Rear-blur priority mode : The DC ring is set to "R" side (Aperture: f2)
When the "incorrect" spherical aberration is set, background light is concentrated in the center of the
subject.
A haze will surround the center and a smooth blur will appear overall.
This is the exact opposite of what occurs when the correct spherical aberration is set: the background and
foreground lighting, in effect, switch roles.
3. Techniques using aperture
In most lenses, there are about seven steps available for setting the aperture.
(As we said in 2.2 fig 3.a. ~ 3.c., even a six (6)-step difference can cause great changes in
the perspective). In order to make full use of aperture characteristics, let's take a look at
two old tricks. Let's also take the time to make an aperture by hand to obtain a special
effect.
3.1. Maximum aperture with a telephoto lens
Here's a technique commonly used in portrait photography. A
telephoto lens has a long focal length, which gives it a shallow
depth of field. By selecting maximum aperture, the depth of
field becomes even shallower, greatly blurring the foreground
and background. Under these circumstances, the subject is the
only true focal point.
Figure 5: right was taken with a 300mmf/2.8 lens set at a
maximum aperture of f2.8.
If the distance to the background and foreground are about the
same - a person leaning against a wall, looking straight on at
the camera, for example - you might not get the desired effect.
The trick is to make a clear distinction between the subject and
the foreground or background, depending on which you want to blur. If you can achieve
the correct distance, you can get some great effects using a standard lens with a focal
length of about 50mm using the 35mm (135) format.
3.2. Minimum aperture with a wideangle lens
Wideangle lenses have a short focal length and a deep depth of field. The absolute
minimum aperture is not particularly necessary, but by decreasing it as much as possible,
you can take a picture in which nearby and faraway objects are both in focus.
The trick is to set the focusing distance between the two objects. Dividing the distances
into thirds is the most effective way to do this: from the camera to the closer subject, 1 / 3
the distance; from the closer subject to the farther subject, 2 / 3 the distance.
If it is difficult to set these proportions using AF (for example, the focus lock does not fix
on the desired subject), then check the depth of field indicator and set it manually.
Fig. 6.
Fig 7.
In order to make use of depth of field, the
focus ring (even if the AF lens does not have
an indicator) should be set in thirds:
Taken with ultra-wideangle lens (AI AF
One-third (1 / 3) the total distance from the
Nikkor 18mmf/2.8D) at minimum
camera to the foreground subject, and twoaperture f22
thirds (2 / 3) the total distance from the
camera to the background subject.
Many AF lenses today do not have a depth of field indicator. However, with a one of the
certain manufacturer, there is an exposure and focusing mode which you can use to work
around this problem. First, aim at the closer object and press the shutter button. Repeat
this step with the farther object. Then, get in position and fully depress the shutter release
button. By doing this, the camera will figure out the appropriate focus and aperture (and
shutter speed).This feature is known as "Depth-of-field priority AE (DEP) mode. Tthis
feature is effective, and be careful when shooting at slow shutter speeds.
Hyper focal distance is the infinite focusing distance obtained within the range of the
longest depth of field possible. If the focus ring is set to this distance, then a wide range
of distances can be put into focus. This is a very effective technique when taking snap
pictures with manual focus. (NOTE: Be careful of camera shake at slow shutter speeds).
3.3. Changing blur with a "handmade aperture"
By positioning a "handmade aperture" at the front of the lens, you can manipulate the
shape of the blur. The blur takes on the shape of the aperture. It is evident from the
sample photos that it's possible to change the shape of the blur coming from illuminated
objects.
The size of the aperture and the lens' focal length may affect the overall size of the final
picture. Confirm how the photo will be framed by checking the viewfinder. Generally,
using a telephoto lens with a long focal length and making the aperture small result in
better photos. Fixing the lens' aperture at the widest setting and shooting in aperturepriority AE mode ((A) mode) will also garner better results.
Samples of "handmade apertures" with different shapes and blurring effects:
Fig 8.a.
Taken with an 85mmf/1.4 lens at maximum aperture (f1.4).
Fig 8.b.
Taken with the same lens using a handmade "cat's-eye" aperture (aperture is f1.4).
Fig 8.c.
Taken with the same lens using a handmade "star" aperture (aperture is f1.4).