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The term Single Lens Reflex (SLR) evolved because
before its conception there was a Twin Lens Reflex (TLR) camera, such as
the well-known Rolleiflex. One lens was for viewing and composing the
shot and the other for taking the shot. Reflex, because the camera uses
a lens to reflect the image to the viewfinder using a mirror. The SLR
works as follows: light enters through the lens and passes to the
mirror. The image is reflected onto a focusing screen where it can be
viewed. When the shutter is released the mirror flips up and out of the
way so that the film or digital sensor is exposed to capture the image.
The greatest advantage with the SLR is that basically “what you see is
what you get”. On digital SLRs there is often a slight difference
between the image seen on the LCD viewing screen to that seen through
the viewfinder. Generally what you see through the viewfinder is
slightly less than that on the LCD screen. SLR cameras are also far
better than their point-and-shoot counterparts as they offer the
photographer far more scope.
Most SLR cameras have interchangeable lenses with far
better aperture control offering a much wider range of adjusting depth
of field (sharpness from foreground to background). Checking depth of
field can be done in two ways: on manual focus you can see how depth of
field is affected as you adjust the aperture. On automatic focus you see
the image as though the aperture is wide open but then closes down to
the aperture setting when the shutter is released. However, most SLRs
have a button to check depth of field when using auto focus, although
the image darkens when the button is pressed.
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