Nikon has filed a patent that makes way for a digital back to convert 35mm film SLRs into the full-fledged DSLRs. The rear door that covers the film holder would be removed and replaced with a digital back equipped with an image sensor.
An excerpt from the patent:
The camera back exchange type camera which images the object image acquired by the photographing optical system by the side of a camera body, and obtains electronic image data is known by replacing with the rear lid with which the camera body was equipped, and equipping a camera body with the camera back having an image sensor.
In the camera back of the description to this Patent document 1, when the moving mechanism to which the position of an image sensor is moved, and the position detection sensor which detects the position of an image sensor are provided and a moving mechanism moves an image sensor based on the output of a position detection sensor, The image sensor to the photographing optical system by the side of a camera body is positioned.