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The aperture diaphragm, a ring of overlapping leaves within the camera
lens, adjusts the size of the opening in the lens through which light passes
to the image sensor. As it changes size, it affects both the exposure of the
image and the depth of field in which everything is sharp.
Aperture and exposure
The aperture can be opened up to let in more light or closed (stopped
down) to let in less. Like the shutter speed, the aperture is used to
control exposure. The larger the aperture opening, the more light reaches
the image sensor in a given period of time. The more light, the lighter the
image.
The Way It Was: Early Apertures
A variety of designs in the past century and a half have enabled
photographers to change the size of the lens opening. A form of the iris
diaphragm, used in today's cameras, was used as early as the 1820s by
Joseph Nicephore Niepce, one of the inventors of photography. Waterhouse
stops, used in the 1850s were a series of blackened metal plates with
holes of different sizes cut in them. To change apertures the
photographer chose the appropriate one and slid it into a slot in the
lens barrel. With wheel stops, different size apertures were cut into a
revolving plate. The photographer changed the size of the aperture by
rotating the plate to align the desired opening with the lens. |
Aperture and depth-of-field
Like shutter speed, aperture also affects the sharpness of your picture,
but in a different way. Changing the aperture changes the depth of field;
the depth in a scene from foreground to background that will be sharp in a
photograph. The smaller the aperture you use, the greater the area of a
scene that will be sharp. For some pictures-for example, a landscape-you may
want a smaller aperture for maximum depth of field so that everything from
near foreground to distant background is sharp. But perhaps in a portrait
you will want a larger aperture to decrease the depth of field so that your
subject's face is sharp but the background is soft and out of focus.
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A shallow depth
of field can make part of an image stand out sharply against a softer
background. This emphasizes the sharpest part of the image. |
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Great depth of
field keeps everything sharp from the foreground to the background. |
Understanding aperture settings
Aperture settings are called f-stops and indicate the size of the
aperture opening inside the lens. Each f-stop lets in half as much light as
the next larger opening and twice as much light as the next smaller opening.
From the largest possible opening to increasingly smaller ones, the f-stops
have traditionally been f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16,
f/22, f/32, f/45. No lens has the full range of settings; for example, the
standard lens on a digital camera will range from about f/2 to about f/16.
Notice that as the f-stop number gets larger (f/8 to f/11, for example), the
aperture size gets smaller. This may be easier to remember if you think of
the f-number as a fraction: 1/11 is less than 1/8, just as the size of the
f/11 lens opening is smaller that the size of the f/8 opening.
How wide you can open the aperture, referred to as its "speed," depends
on the lens' maximum aperture (its widest opening). The term "fast lens"
usually applies to lenses that can be opened to a wide maximum aperture for
the focal length. For example, a lens with a maximum aperture of f/2.6 opens
wider, and is faster, than a lens with a maximum aperture of f/4. Faster
lenses are better when photographing in dim light or photographing fast
moving subjects. With zoom lenses the maximum aperture changes as you zoom
the lens. It will be larger when zoomed out to a wide angle, and smaller
when zoomed in to enlarge a subject.
How To: Selecting an Aperture
Look in your camera manual for a section on aperture
preferred or aperture priority, or
apertures. |
Both shutter speed and aperture affect the exposure, the total amount of
light reaching the image sensor, and so control a picture's lightness or
darkness. The shutter speed controls the length of time the image sensor is
exposed to light and the aperture controls the brightness of that light.
You, or the camera's autoexposure system, can pair a fast shutter speed (to
let in light for a short time) with a wide aperture (to let in bright light)
or a slow shutter speed (long time) with a small aperture (dim light).
Speaking of exposure only, it doesn't make any difference which of the
combinations is used. But in other ways, it does make a difference, and it
is just this difference that gives you some creative opportunities. You're
always balancing camera or subject movement against depth of field. This is
because a change in one causes a change in the other. Let's see why.
Each setting is 1 "stop" from the next and lets in half or twice the
light of the next setting. A shutter speed of 1/60 sec. lets in half the
light that 1/30 sec. does, and twice the light of 1/125 sec. An aperture of
f/8 lets in half the light that f/5.6 does, and twice the light of f/11. If
you make the shutter speed 1 stop slower (letting in 1 stop more light), and
an aperture 1 stop smaller (letting in 1 stop less light), the exposure
doesn't change. However, you increase the depth of field slightly and also
the possibility of blur.
For general shooting you need a medium shutter speed (1/60 sec. or
faster) and a medium aperture (f/5.6 or smaller). Slower shutter speeds will
show up on the image as overall blur unless you support the camera, perhaps
with a tripod.
- For fast-moving subjects you need a fast shutter
speed (although the focal length of the lens you are using, the closeness
of the subject, and the direction it's moving also affect motion).
- For maximum depth of field, with the entire scene
sharp from near to far, you need a small aperture (although the focal
length of the lens and the distance to the subject also affects depth of
field).
An Analogy
One way to think of shutter speeds and apertures is as faucets. You
can fill (expose) a bucket with a small faucet opening (aperture) over a
long time (shutter speed), or a large faucet opening in a shorter
period. No matter which combination you choose, the bucket can be filled
the same amount. |
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Photographing
these fast-moving Blue Angels from the deck of a moving boat took a fast
shutter speed to prevent blur caused by subject or camera movement.
Great depth of field was also needed to keep the boats in the foreground
and background sharp. |
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