Recent photographic studies have generated renewed interest in the optics of the retinoscopic (skiascopic) image. In photoretinoscopy the subject's eye is illuminated by a point source of light and the fundal image of this light at the plane of the subject's pupil is observed or photographed from a position near the source. The image so obtained is a function of the dioptric defocus of the eye relative to the camera, D, the distance of the source and camera from the eye, A, the radius of the subject's pupil, R, and the distance of the point source from the edge of the camera aperture, E. The fraction of the subject's pupil which is not filled with light is termed the "Dark Fraction" (DF) and may be computed from a simple ray tracing model of the eye. It is given in terms of the above parameters by the equation DF = E/(2ARD). The validity of this equation was tested with a photoretinoscope and artificial eye. The photoretinoscope, which is a novel modification of an isotropic photorefractor, is also described.