A hinged mirror behind the lens reflects the image up into the viewfinder. When the shutter release is pressed, the mirror swings up out the image path and the shutter opens, exposing the image onto film. When the shutter closes,the mirror swings down into viewing position again. What you see in the viewfinder is what you get on film and lens focus is easy to judge and set. Lenses can be interchanged simply and quickly, and a wide selection of moderately priced wide-angle, telephoto and zoom lenses are available.
Many cameras have a built-in motorized film winder, while others accept a separate autowinder.
The latest advance in SLR cameras is autofocusing. A microcomputer and focus sensor in the camera adjust the lens focus quicker and more accurately, in most cases than the photographer can manually. Many SLR's have a built-in electronic flash, plus a hot shoe on top of the viewfinder for attaching a separate, more powerful flash to the camera. Most cameras accept dedicated flash units, which means that both camera and flash have a few special electronic contacts on the hot shoe that mate together. Both conventional and dedicated flashes are available in a wide range of sizes and prices.