The effect of spherical and other aberrations upon the modulation transfer of the defocussed human eye.

Relative modulation transfer is defined as contrast sensitivity under blur normalised to contrast sensitivity at optimum focus. Measured relative modulation transfer exceeds relative modulation transfer as calculated for aberration free optics at higher spatial frequencies (> 2 cpd). The contribution of spherical aberration to this discrepancy was studied. In the case of positive blur, spherical aberration increases relative modulation transfer for spatial frequencies of approximately 4 cpd and over. However, spherical aberration does not explain the entire difference between measured and calculated relative modulation transfer. Chromatic aberration does not provide a noticeable additional explanation. With negative blur, spherical aberration increases the relative modulation transfer much more. The effect of other monochromatic aberrations on relative modulation transfer is discussed.