Of cameras: Using a single lens for both framing and recording photographs, allowing the user to see the view through the camera lens in the viewfinder. Frequently abbreviated as “SLR. ” A reflex mirror is positioned between the lens and the film or image sensor, which reflects the light from the lens to the viewfinder for framing and focusing. When a photograph is taken, the mirror is raised to expose the film or image sensor. Most SLR cameras use a pentaprism prevent the image in the viewfinder from being reversed. “Single-lens” refers to the fact that a single objective lens is used for both the viewfinder and photography, while “reflex” is used in reference to the reflection of light by the mirror.