Best Focal Planes Determined by Different Image Quality Criteria

Abstract

Relative contrast values that correspond to the best focal planes defined by minimizing the different image quality criteria are examined in the whole frequency range. The quality criteria are those commonly employed as merit functions in automatic optical design: the wavefront variance, the variance of the difference function and the geometrical r.m.s. spot size. Analogous results obtained when the diffraction-based modulation transfer function (MTF) is directly maximized by numerical searching for the true best focal plane at a given spatial frequency, are also computed. The results show how the image quality criteria correlate with the true frequency response for different states of correction at various spatial frequencies. Since odd aberrations do not change the localization of the best focal places defined by the three criteria chosen for consideration, the spherical aberration case is selected for detailed investigations. A number of conclusions of direct importance in optical design and testing ar...