The hottest thing in the digital camera market is undoubtedly the
digital SLR, which is better known as a dSLR. While dSLRs are flying off
dealer shelves, many new users are confused about the terminology. Most
people know that SLR stands for "single lens reflex." Since nearly all
SLRs accept interchangeable lenses, it would appear they should be known
as multiple lens reflex (MLR) cameras.
If you want to understand
how the SLR received it's name, you have to dip into the history of the
camera. Early cameras were similar to the view cameras used today. The
photographer looked through the lens, focused, composed and then
inserted a single film plate behind the optics to make an image. While
the entire process was crude by modern standards, the photographer
enjoyed great control, since he looked directly through the actual
imaging lens to compose the shot.
While this was fine for still
life, portraits and landscapes, this process did not lend itself to
action photography. These early cameras could only record a single image
at a time. Which is why you have never seen a motor-driven view camera.
Realizing
the need to offer sequences of exposures, camera makers begin to
experiment with various roll-film designs. With a roll of film in the
camera, the photographer could fire off numerous images without
reloading. Although this improved throughput dramatically, it caused
another problem. The roll of film had to pass closely behind the
camera's optics, which meant that the photographer could no longer look
through the camera lens to compose and focus.
Rangefinder cameras appear to keep things in focus
The
lower-end, consumer roll-film cameras generally used an inexpensive
"fixed-focus" lens, so a simple viewfinder was sufficient. Better
quality optics, however, require the lens to be focused, and since the
photographer could not look through the lens with a roll-film camera,
this was a major problem. One of the first solutions to this problem was
the Rangefinder -- a type of camera that offered a distance measuring
scale in the viewfinder. By determining the range from the viewfinder,
the photographer could then adjust the focus to match -- usually with
very good results.
Twin Lens Reflex cameras offer another solution
While
the rangefinder type cameras worked well, the camera industry is always
evolving. A second method of allowing the photographer to focus and
compose appeared in the "Twin-Lens Reflex" cameras. These cameras used
two identical lenses, arranged one on top of the other in the manner of
an over-and-under shotgun. The film winds past the lower lens, while the
photographer can focus through the upper lens. The twin-lens cameras
were fairly bulky, so designers added a mirror and ground glass to the
top of the camera, hence the term "reflex.
Now the user could hold
the camera at waist level and look down at the ground glass which
previewed the image via the mirror behind the upper lens. As the user
adjusted the focus on the upper lens, a gear mechanism moved the lower
"taking lens" to match.
While both rangefinders and twin-lens
reflex cameras offered a credible way to focus and preview a shot,
neither allowed the photographer to actually look through the lens. This
sometimes made exact composition difficult.
SLRs take cameras another step forward
In
their quest to allow users to see through the actual "taking" lens,
camera makers turned to the periscope -- a simple device using two
mirrors placed at opposite angles to bend the light path. Periscopes are
easy to understand -- any kid can construct one from a couple of
mirrors and some scrap wood.
In a camera, the lower mirror is
placed at a 45 degree angle directly behind the lens. Light striking the
mirror is projected upwards to a ground glass. While a second mirror
would show the image on the ground glass to the user, it would not
appear right, because mirrors tend to reverse things. So camera
designers added a prism arrangement that corrects the reversed image.
When you peer through the viewfinder on a SLR, you look through a prism,
which displays the image on a ground glass, which displays the
projected image from the mirror located behind the lens.
There is
just one problem. If you have been paying attention, you have no-doubt
realized that the lower mirror blocks the light path to the film (or
digital sensor as the case may be.) Now the photographer can look though
the lens, but the image cannot be projected on to the filmplane.
So
the camera designers had to add another wrinkle. They had to move that
mirror. Just long enough to make an exposure, since when the mirror
moved, the photographer could no longer see anything through the lens.
So they designed the "instant-return" mirror. At the instant of
exposure, the mirror flies upward, the shutter fires and the mirror
snaps back down. It is a incredible feat, when you consider that instant
return mirrors have to flip up and back in a heartbeat, over and over
for the life of the camera.
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