Abstract: A slit image is projected through the optical device being measured and onto the optical input screen of a variable-gain photomultiplier tube. Before it is incident upon the input screen of the tube, the projected image is modulated to develop simultaneously a low-frequency image component and one or more higher-frequency image components. Electronic filters are employed to separate the electrical output signal of the photomultiplier tube into discrete electrical signals each representative of a different one of the image components, and one or more meters are used to indicate the relative amplitudes thereof. A phase-locked-loop detector and an amplifier are employed between each filter and meter to improve the signal-to-noise ratio of the system and increase the signal amplitude to a level suitable for driving the meter. In one embodiment, the overall gain of the system is manually adjusted until the meter reads 100 percent MTF for the low-frequency component.