Single Lens Reflex Camera
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Probably the most widely used type of
camera today is the single-lens reflex (SLR). Most single-lens reflex
cameras use 35 mm film, but several popular makes use 21/4 inch film
sizes, and a few use 4 x 5 or even 5 x 7 inch film sizes. The
single-lens reflex camera is so called because the lens used to make
the picture also serves, through the interposition of a moving mirror,
as rangefinder and viewfinder system. When a scene is viewed and
brought into focus, the light from the lens is directed to a viewing
screen by the mirror, and from there to the eye by way of the prism.
The prism reverses the image reflected from the mirror into the correct
left-right configuration, and allows eye-level viewing as well.
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In modern single-lens reflex cameras,
pressing the shutter release sets off a train of mechanical activity:
the mirror swings up out of the light path and the focal plane shutter
opens, which allows the image to be recorded on the film. In most
modern designs, the iris diaphragm in the lens mount is "set" at the
desired opening prior to focusing but actually remains wide open until
the shutter release is pressed. Then, just before the shutter opens,
the iris automatically closes down to the chosen setting for the
duration of the exposure. In most cameras, the mirror comes back down
and the iris reopens automatically as soon as the shutter closes, to
restore viewing at once. Thus, direct viewing of the subject is
interrupted only for a split second longer than the duration of
exposure, allowing virtually constant monitoring of subject activities.
Although single-lens reflex cameras have the most complicated
mechanical design of any common camera type, nearly all of them are
remarkably dependable.
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They are also highly versatile. Since
viewing and picture making are accomplished with the same lens, there
is no parallax problem. Extreme close-up and very long telephoto
photography are readily accomplished and there is wide
interchangeability of lenses. Meanwhile, the basic camera approaches
the rangefinder camera's small size, lightness, and speed of operation
(indeed, most 35 mm versions have a small variation of the split-image
rangefinder built into the viewing screen to speed up and assist
focusing). Some manufacturers have recently introduced unusually small
and compact 35 mm SLR models, and a general trend toward smaller size
is now apparent. SLRs using films as small as the 110 size are
available, including models with interchangeable lenses.
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As for disadvantages, the single-lens
reflex design is slightly more conspicuous in use owing to the combined
noise of the focal plane shutter and the moving mirror, and in a few
very critical uses "mirror-flop" can introduce unwelcome vibrations.
(These latter will be most detectable if slow shutter speeds are being
combined with high magnification, as with extreme telephoto
photography.) In a few SLRs the mirror can be manually locked up out of
the path of light prior to exposure to avoid the problem. This is
useful when the subject is motionless.
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Single-lens reflex cameras using 35 mm or
smaller films share with 35 mm rangefinder cameras the disadvantage of
small film size. However, the better designs partially compensate for
this by very exact framing characteristics, which allow absolutely full
use of the available film area, an essential feature if color slides
for any critical or important use are to be made.
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The special subject areas in which the 35
mm single-lens reflex camera excels are photojournalism and nature
photography of small, live, moving objects, such as insects. As an
indication of the popularity and versatility of single-lens reflex
cameras, approximately 90% of all photojournalists now use this type of
instrument-as does probably a similar percentage of all serious
photographers.
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