As shown in this illustration, if an imperfect eye views through a disk which has two pinholes, a single distant point of light such as a star will form two retinal images. If the eye's imperfection is a simple case of defocus, then the double retinal images can be brought into register by viewing through a spectacle lens of the appropriate power. Thus the Scheiner Disk was the first optometer for measuring refractive errors of eye.

A simple lens won't always bring the two retinal images into coincidence, however, so a more general method is needed for quantifying the refractive imperfection of the eye at each pupil location.