ABSTRACTIt is possible of course to locate the image plane of an optical system with high precision by the method of auto‐collimation. It frequently happens, however, that the image plane of the system in situ is not readily accessible, or perhaps the problem does not call for the precision and consequent trouble of an auto‐collimation assembly. In these cases a method is described of locating the image plane with tolerable accuracy, using the principles involved in retinoscopy, which is a routine procedure in optometric practice. The method can be usefully applied in camera optics. It can be used in any case where the positions of a pair of real conjugate points have to be determined, with the stipulation that the accuracy becomes lower as the conjugate distances tend to become more nearly eq