Definition: SLR; Single Lens Reflex Camera;

SLR; Single Lens Reflex Camera;

The SLR or Single Lens Reflex camera is an old format of camera originally designed to take film. The SLR provides viewers with a line of sight through the main lens. As Light passes through the photographic lens the image is inverted and reversed by the lens. Next the image hits the ‘reflex’ mirror and is directed up into the viewfinder. There it passes through a pentaprism. The pentaprism is a lens which corrects the image by flipping it back in the horizontal and vertical dimensions and directs it to the viewers eye.

Diagram two: Side view of an SLR camera showing the optical path.
Light passes through the lens which inverts and reverses the image. The pentaprism corrects that and directs the reverted image to the photographers eye.
The shutter normally lies behind the mirror and in front of the film. It is not shown here for simplicity.
(Click to view large).

The main mirror flips up when the camera shutter is opened. Thus, for the brief opening, the photographer’s view out of the camera is interrupted. This is because the new optical path projects the image directly onto the film plane where the film is exposed.

Modern DSLRs have the same system as the old film SLRs except the film and film plane are replaced by the digital image sensor at the sensor plane.

A more in-depth discussion of the SLR can be found in What is an SLR?

See a detailed description of a Digital Single Lens Reflex Camera (DSLR).

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