The single-lens reflex (SLR) camera uses an automatic moving mirror system which permits the photographer to see exactly what will be captured by the film or digital imaging system, as opposed to non-SLR cameras where the view through the viewfinder could be significantly different from what was captured on film.
Prior to the development of SLR, all cameras with viewfinders had two optical light paths: one path through the lens to the film,and another path positioned slightly off to the side seen by the photographer. Because the viewfinder and the film lens cannot share the same optical path, the viewfinder is aimed to intersect with the film lens at a fixed point somewhere in front of the camera. This is more or less helpful for pictures taken at a middle or longer distance but a close-up shot framed in the viewfinder will not look the same as the film image. Moreover, focusing the lens of a non-SLR camera when it is opened to wider apertures (such as in low light or while using low-speed film) is not easy. More…
