An analysis on the equivalence of the eye to a system with aberration

Computed transfer functions for epsilon = 0.60 and 0.75 annular pupils of 2.67 mm o.d. show that absorption of the average spherical aberration of the eye is still incomplete but if the eye is so designed as to adjust its focal plane in order to achieve a best Strehl ratio, it would show a transfer function approaching that of a diffraction limited system, i.e. the optical defect of the eye is overcompensated. However, it is argued that a varying amount of focusing adjustment of the eye is needed in order to account for the transfer function of the eye of different pupil sizes, though not always to the extent of achieving a best Strehl ratio. Such a property of the eye is referred to variously as the antidefocusing, deblurring or contrast constancy phenomenon and, when operationally defined, this is equivalent to saying that the eye has a greater depth of focus than that defined for a physical system based upon a fixed demodulation criterion. The significance of the physiological depth of the eye is discussed.