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.