Comparison of Lens Response for Sinusoidal and Square-Wave Targets at Several Focal Positions

A study has been made of the sine-wave and square-wave responses of a lens at two apertures and several focal positions, both on- and off-axis. Two focal positions, one of which gives the best definition and the other the highest contrast for coarse patterns, were located precisely. At these focal positions and at several other arbitrarily chosen positions the sine-wave and square-wave responses were measured. Because of the scattering character of the photographically-made target objects, a special technique has been employed for calibrating the targets to obtain their contrast as seen by the lens under test. The results show that for large apertures of the lens there is some frequency at which the response (either sine or square) is the same for the two focal positions (curves cross). This phenomenon has been further studied by computing the square-wave response of the lens from its sine-wave response; and it was found that there is close agreement between the computed and experimentally determined responses.