orthoscopic turns on and off the perspective handling. Turning on orthoscopic rendering removes the effects of perspective. To adjust perspective rendering see, the Field of View command.
Turning on orthoscopic rendering improves the rendering speed by a factor of 2x-4x! So, if you have large images, or are rendering a movie that does not need perspective in the images, try turning off perspective by setting orthoscopic to 1.
For many images, the difference is hardly visible
Especially when straight features parallel to the z axis are shown, the effect may be large
set orthoscopic, on set orthoscopic, off