An airplane is taking pictures of the ground. Its camera is facing straight down. The camera is loaded with a long roll of film that is slowly advanced by a motor.

Come up with a system that controls the rate of advance so that the film moves at the same speed as the image projected by the lens, regardless of airplane speed and altitude.

Complication: only use technology available in 1950’s. Photosensors and basic electronics is ok, but no CCD and no computers.

One possible solution: place a semitransparent mirror between the lens and the film to project image on photosensor. Cover photosensor with a grating that consists of equidistant opaque lines perpendicular to the direction of travel. Measure frequency of the signal from photosensor and use that to control the motor.

Source: probably dz.livejournal.com, but can’t find the direct link.