Transposition

Transposition in stereo photography is the process of swapping the left and right images of a stereo pair. This operation reverses the horizontal order of the images. An image captured by a stereo camera needs to be transposed because the optical path of the camera reverses the left–right relationship of the captured images.

To produce correct stereoscopic depth, the image intended for the left eye must be viewed by the left eye, and the image intended for the right eye by the right eye. Transposition—swapping the left and right images during printing—restores the correct binocular disparity. Without transposition, the stereoview would produce reversed depth (pseudoscopy).

Transposition can be challenging in the case of autochrome. Autochrome is a direct positive process in which the negative is developed directly into a positive image. To transpose an autochrome, the plate must be cut, after which the two halves can be swapped.

There are stereoscopes, such as Jules Richard’s Stéréoscope redresseur, that achieve the swapping of the two images optically, so that transposition is not required.