Photoelectric measurement of aerial image modulation at a single spatial frequency has been applied to the focusing of lenses in aerial cameras. Using an automated apparatus to provide a plot of modulation through focus, the settings are made in a fraction of the time and with much greater confidence than by the photographic resolving power method. It is pointed out that the focus for a single well-chosen frequency can be a good index of the focus for all frequencies in the photographically-important region of the MTF of the lens, also that a through-focus plot of modulation at one or two important frequencies is often of greater value to the camera manufacturer than the usual MTF plot at one focus position. The well-known difficulty in focusing some lenses by high contrast resolving power is explained in terms of their decreased sensitivity to focus setting above a certain frequency; in some cases this effect cannot be eliminated by moderate stopping down. Although primarily designed for measuring relative modulation on axis, the apparatus can be calibrated to read absolute modulation to a useful degree of accuracy, can be used off-axis, and has many photo-optical applications.