SLR, which means single lens reflex refers to a type of device that uses a rotating mirror (either a pentaprism or pentamirror) which reflects the image that comes through the lens onto a screen development. Reflection mirror, the image appears in the eyepiece of the camera. The image does not touch the film after the focal plane shutter opens (when the photographer took the shot).

While digital SLR cameras give the photographer the advantage of watching the scene without distortion of parallax (the apparent movement of stationary objects when changes photographer position), they prevent the photographer to see his shot when the photo was taken.