Electrostatic copying machine

TL;DR: A variable magnification optical system focusses an image of a portion of an original document on a platen through a variable aperture or slit onto a rotating photoconductive drum as mentioned in this paper.

Abstract: A variable magnification optical system focusses an image of a portion of an original document on a platen through a variable aperture or slit onto a rotating photoconductive drum. Various members of the optical system or the document are moved at a speed corresponding to the surface speed of the drum to scan the document and form an electrostatic image thereof on the drum. The slit is defined by a fixed aperture plate and a movable aperture plate disposed upstream of the fixed aperture plate in the direction of rotation of the drum. A variable transmission connected between an exposure knob and the movable aperture plate transmits rotation of the knob to the movable aperture plate at a rate corresponding to the magnification. In addition, a side edge of the document is aligned at the same position for all magnifications. The movable aperture plate is formed with a curved edge facing the fixed aperture plate, the center of the curved edge being aligned with the center of the image at all magnifications. As end plate is positioned by a sheet feed mechanism to prevent image formation on a portion of the drum in which the image would not be transferred to an undersized copy sheet.