Lateral reversal is a photographic and optical term describing the mirror image transposition of a subject from left to right - or right to left - that occurs when light is reflected from a flat mirror or passes through certain optical systems. In practical photographic terms, lateral reversal is most commonly encountered when viewing the image on the ground glass screen of a large format camera or twin lens reflex camera, where the image appears correctly oriented in terms of up and down but is reversed horizontally, so that elements appearing on the left side of the actual scene appear on the right side of the viewed image and vice versa.
The lateral reversal seen in the viewfinders and ground glass screens of certain camera types is a direct consequence of the optical path the light follows through the instrument. In a twin lens reflex camera, light entering through the viewing lens is reflected upward by a fixed mirror onto the ground glass screen on the top of the camera, producing an image that is the correct way up but laterally reversed. Similarly, when viewing the ground glass at the rear of a large format camera, the image projected by the lens onto the screen is both upside down and laterally reversed, requiring the photographer to mentally compensate for both orientations when assessing composition and framing.
In single lens reflex cameras, the pentaprism or pentamirror viewfinder system corrects both the vertical inversion and the lateral reversal of the mirror image, presenting the photographer with a correctly oriented view of the scene that matches what they see with the naked eye. This correction is one of the primary practical advantages of the SLR viewfinder system over the simpler mirror arrangements used in twin lens reflex and large format cameras.
Lateral reversal also has relevance in the darkroom context, where contact printing a negative in direct contact with photographic paper will produce a print that is laterally reversed relative to the original scene unless care is taken to ensure the negative is oriented with its emulsion side correctly positioned against the paper. In the production of printing plates and certain photomechanical reproduction processes, lateral reversal must be carefully managed to ensure that text and images are correctly oriented in the final printed output.