Determination of the optimum double-pass image through focus operators.

A set of autofocus operators (AFO) are assessed for their ability to determine the optimal double-pass image as well as their potential to detect the focus lines and the disc of least confusion in astigmatic eyes. Eight AFO and three optical quality parameters (OQP) often used to analyze double-pass aerial images were considered. To quantify the discriminative power of each AFO and OQP, a maximum discrimination (MD) parameter was proposed. Double-pass images were obtained from an artificial eye with an induced astigmatism (Cylinder: -1  D, -0.75  D, 0.75 D, 1 D) and without astigmatism (Sphere: 0.1 D) and from 19 eyes of subjects with different refractions. The MD values for the autofocus operators Tenengrad variance and Gray level local variance were the highest for the artificial eye with and without astigmatism. In the case of astigmatic eyes, the discrimination of the focus lines with the autofocus operator Tenengrad variance was better than with OQP.