Chromatic and monochromatic optical resolution in the rainbow trout

The modulation transfer function due to measured longitudinal chromatic aberration was calculated for the otherwise unaberrated eye of the adult rainbow trout (Oncorhynchus mykiss) under daylight conditions assuming light absorption by single retinal cone pigments, and by photopic mechanisms involving interaction between cones. The adult trout eye, with its large immobile pupil, is limited by chromatic aberration to resolution much lower than the diffraction limit, consistent with the low acuity reported for fish. This low resolution can be considered a design trade-off cost of a bright image. The measured monochromatic modulation transfer function is similar to that calculated due to chromatic aberration alone, showing that these independent aberrations are approximately balanced in the fish eye. The effect of changes in receptor length, pigment density, water depth, and pupil size upon the chromatic resolution was calculated. The calculated chromatic modulation transfer function will hold approximately for other teleost eyes with lens larger than about 1 mm.