Optical factors in judgments of size through an aperture.

An expression is derived relating retinal image size to object size when the object is seen through a pupil forming an aperture stop for the eye. The image size is related to object position, pupil position, and state of accommodation of the eye on the assumption that the chief ray determines the image of an out-of-focus object. Data obtained by Biersdorf and Baird agree with the expression. The expression is used as the basis of a quantitative discussion of Hennessy's informal experiment and Roscoe's zoom lens hypothesis.