Abstract: For measuring the optical transfer function of an optical instrument having an input pupil, a device comprises a source of spatially coherent light for illuminating the pupil of the instrument. Two separate zones are isolated in the input pupil of the instrument by a system comprising a grating, a lens for forming an image and a spatial filter which eliminates the zero diffraction order. The two zones have a spacing which is in direct relation to the spatial frequency. The position and mutual spacing of the fringes are determined. A computer derives the transfer function from the measurements made for a plurality of pairs of zones.