indicated on the lens at all. You set the f-stop on these lenses by rotating
the control wheel, a dial on the camera body, until the desired f-stop is
displayed on the LCD panel and/or in the camera’s viewfinder.
The f-stops on the opposite page have a special relationship to each other,
one that is critical for understanding film exposure and how to control it.
Changing the lens aperture setting from one of the f-stops in this list to one that
comes just before or after it halves or doubles the amount of light the lens
allows through, depending on whether you make the opening smaller or larger.
For example, changing from f/8 to f/11 makes the lens aperture half the size, so
it lets in half the light. Changing the lens aperture by two f-stops from f/8 to
f/16 reduces the light to one-fourth. Conversely, opening the lens from f/8 to
f/5.6 doubles the amount of light let in and opening it to f/4 allows in four
times the amount of light.
Note that these f-stops are sometimes known as whole, or full, f-stops. But not
all lenses offer the full range of these stops. Some don’t open as wide or close
as much, while others open wider and close more. For example, one lens may
have a maximum aperture of f/2, while another only opens to f/4. Still another
lens may have f/22 as its smallest aperture, while another only f/16.
Still others don’t offer whole f-stops as their maximum lens aperture, possibly
opening up to f/3.5 instead of f/2.8. In this case, f/3.5 is a partial stop,
meaning it is a setting in between two whole f-stops (smaller than f/2.8, but
larger than f/4). For more precise exposure control, you can deliberately set
lenses in between whole f-stops. On lenses that permit you to set the lens aperture
by turning a ring on the lens barrel, you simply turn the ring until the
marker points in between two whole f-stop settings. On many newer camera
models, the partial f-stops are shown in an LCD display. In-between settings
are indicated in half stops (halves) or third stops (thirds), depending on the lens
or camera system.
Some lenses are described as fast and others as slow. A fast lens has a large
maximum aperture, such as f/1.4, f/1.8, or f/2. Such lenses are capable of
allowing a lot of light in to reach the film, making them excellent choices for
low-light conditions, such as outdoors at night or indoors; these lenses also
allow for faster shutter speeds to capture subjects in action. A slow lens has a
smaller maximum aperture, perhaps f/3.5, f/4, f/4.5, or f/5.6. Such lenses don’t
let as much light in, so they require bright light conditions or auxiliary lighting,
such as a flash; otherwise, they may require slow shutter speeds and possibly a
tripod to steady the camera.
Lenses made for SLRs stay wide open (at their maximum aperture), regardless
of what f-stop you choose, until you actually take the picture. An f/2 lens,
for example, will remain open to f/2 even if you set the lens aperture to any
other f-stop in preparation for your shot. The lens and shutter are coupled, so
when you press the shutter button, the lens automatically closes down to the
selected f-stop for the correct exposure, and then the lens instantly opens up
again to its maximum aperture until you press the shutter button for the next
picture. This guarantees that the viewfinder will show the brightest possible
image for easiest viewing and focusing, since the most possible light passes
through the lens.
All things being equal, fast lenses are preferable to slow lenses. Not only
do they work better under lower light levels, they also make the subject look
brighter, which makes it easier to see and focus with any camera that has
through-the-lens viewing, such as an SLR.
Aside from helping to control focus and film exposure, a camera lens also
controls the angle of view, or how much of the scene the lens sees from camera
to subject. Some lenses take in a narrow view of the subject while others see a
normal or wide view. A special category of lenses, called zoom lenses, can see a
range of angles.
Most lens types break down into these categories reflecting different angles
of view: normal, wide angle, and telephoto. What makes a lens normal, wide,
or telephoto is directly related to its focal length. The shorter the focal length,
the more of the subject the lens sees.
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