SLR stands for “single-lens reflex.” It is a term used to differentiate this type of camera from basic point-and-shoot cameras or TLR -- twin-lens reflex -- cameras. SLR cameras give more control over exposure than a point-and-shoot, but only use one lens to focus and capture an image. They are a basic starting point for anyone who wants to learn how to use a camera manually.
Pentaprism and Viewfinder
Point-and-shoot cameras can become problematic when photographing subjects close up. The fact that the viewfinder is on one side of the camera body but the lens is in the center creates shifts in composition. The SLR camera fixes this issue by using a pentaprism. Light enters the camera through the lens and is bounced off a series of mirrors in the pentaprism, displaying in your viewfinder the image in its precise composition of how it will be captured.
Unlike a point-and-shoot, SLRs allow for sharper focusing. The most basic SLRs may only have manual focus and employ a microprism or split-focusing aid to help with focus. More advanced SLRs have autofocus, which is activated by tapping the shutter release button halfway. Most cameras automatically focus on what is in the center of the composition, though some others provide options to focus on other areas or to even keep the focus trained on moving objects.
SLRs are built to accept a variety of lenses. Many cameras come with a fixed 50 mm lens, considered a “normal” focal length for a 35 mm SLR. Another standard is a 28 to 80 mm zoom lens that allows for both wide-angle and magnified compositions. Extremely wide-angle lenses, called fisheye lenses, are available to create spherical images.
SLRs let you choose your aperture setting through either a manual mode or a partially automatic mode. Aperture choice affects how much of the depth of your composition will be in focus. Basic SLRs list the choices in a ring around the lens where it meets the camera body. How wide your aperture will open, and thus how fast of a shutter speed you can use, is linked to your chosen lens. “Faster” lenses, due to their apertures, are more expensive than standard ones.
Shutter speed is controlled through the SLR camera body. It controls how long the shutter lets in light and is capable of capturing motion blurs. Many cameras have a Bulb setting, marked by a “B,” that lets you keep the shutter open as long as you hold down the shutter release button. Basic cameras provide the fastest shutter speeds of about 1/2000 of a second, though more expensive models can go even faster.