The effect of the size of the small aperture which isolates the central portion of the interference fringe system in the photoelectric scanning Fabry‐Perot interferometer has been investigated. The phase shift due to size of aperture is given accurately by &Dgr;=K&thgr;2/2, whereKis half the phase difference and &thgr; the angular radius of the aperture, only if the diameter of the aperture is not greater than about one‐eighth of the diameter of the first interference ring. This phase shift is equivalent to a length correction with the Fabry‐Perot interferometer of &thgr;2/4 per unit length. This simple relation cannot be used with larger apertures, and visibility considerations make the use of such apertures quite undesirable.