Short-focus lens

A short-focus lens has a relatively small focal length and provides a larger angle of view at a shorter viewing distance. Such lenses were used to address a limitation in tray-based stereoscopes designed for the small 45 x 107 format. The distance between standard lenses and the small stereoviews resulted in a reduced apparent image size with a large amount of surrounding dark space from the stereoscope housing. Short-focus lenses reduced this effect, although they could introduce image distortion.