Abstract

Uncorrected spherical and astigmatic defocus are the most significant sources of retinal image degradation in human eyes. When sufficient defocus is added to an otherwise diffraction limited optical system, the modulation transfer function (MTF) drops quickly from 1.0 at zero spatial frequency and becomes zero. Beyond this first zero, the MTF oscillates and additional zeros occur (Figure 1a). Each time the MTF passes through zero there is a phase shift of half a cycle. When spherical aberration is added to defocus, the shape of the MTF changes (Figure 1a). If the spherical aberration has the same sign as defocus (in wave aberration terms), the zeros in the MTF will-occur at lower spatial frequencies. The reverse is true if the signs are opposite. The dips and rises in the MTF are more marked when defocus and spherical aberration have the opposite sign than when have the same sign. Other aberrations or combinations of aberrations can cause the MTF to rise and fall without necessarily passing through zero, with shape and magnitude of the MTF often dependent on the object orientation (Figure 1b).

© 1996 Optical Society of America