Single lens reflex cameras allow the photographer to see the precise focus, color, framing, and brightness that will be recorded on film, because the viewfinder looks through the lens. Artistic or professional photographers prefer SLR cameras because they have more control by manually defining their image versus point-and-shoot or range-finder cameras. A prism, flip-up mirror, and interchangeable lens makes an SLR camera extremely accurate.
The light from a scene follows a different path in an SLR camera compared to an ordinary point-and-shoot camera. A mirror behind the lens reflects the incoming image to a corrective prism. The prism turns the image upside-down, since a lens always reverses the orientation of a picture, so the photographer perceives it as upright. The prism may also slightly sharpen and brighten the image. Finally, the image reaches the rear of the camera where the photographer can examine it. After the photographer clicks the shutter release button, but before the shutter opens, this mirror flips out of the way, allowing the light to continue on to the film.
Another reason SLR cameras are more exact is that they utilize through-the-lens, or TTL, metering. The light meter is behind, not in front of, the focusing lens; this allows the meter to monitor the amount of light that will actually hit the film surface, rather than monitoring the light hitting the front of the lens. When filters or masks are screwed onto the lens, these additions will change the light meter's readings, which wouldn't be true for a non-SLR camera.
The use of interchangeable lenses is made possible by positioning the shutter directly in front of the film, rather than including it with the lens. A photographer can easily switch between lenses in the middle of a roll without exposing the film. Some special lenses are macro, fisheye, wide-angle, and zoom. Sophisticated lenses are even compatible with microscopes and telescopes.
SLR cameras eliminate parallax error that is inevitable with cameras that have different paths for the lens and the viewfinder. Parallax is the phenomenon by which the shorter the distance between the subject and the camera, the greater the angular error in the actual and the perceived image. In point-and-shoot cameras, not only is parallax a problem, but the camera guesses at depth of field, shutter speed, and focus, without the photographer being able to decide or intervene. Of course, some SLR cameras have automatic focus, but most of their settings can be controlled manually, which allows a photographer to get a good shot in fast action, a back-lit subject, or an unusual focal plane.
