A wide-angle gradient index optical model of the crystalline lens and eye of the octopus.

Cephalopods and fish have had no common ancestor since the Cambrian, and their eyes are a classic example of convergent evolution. The octopus has no cornea, and immerson renders the trout cornea optically ineffective. As a result, the nearly spherical lens is responsible for all refraction in these eyes.

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.

Image formation by the crystalline lens and eye of the rainbow trout.

The image of a distant unresolved point (point image or PI) and modulation transfer function (MTF) of the eye and lens of the trout were recorded with high spatial (0.3 micron) and dynamic (4096 grey levels) resolution for various entrance aperture sizes and focal positions in monochromatic light, and in broadband light simulating sunlight absorbed by a retinal cone pigment. The PI is irregular, with streaks, wisps and speckle, as a result of lens structural irregularity and diffraction of light scattered within the lens and cornea.

A wide-angle gradient index optical model of the crystalline lens and eye of the rainbow trout.

Trout lens external shape and internal refractive index gradient structure were measured and used to construct an optical lens model that predicts by ray tracing the average longitudinal spherical and chromatic aberration, focal length and image quality. The nearly spherical shape of the lens was measured from photographs, and the internal refractive gradient structure was measured directly with a special Pulfrich areal refractometer. Longitudinal spherical aberration and back focal length were measured using a simplified Hartmann test using laser beams and a Schlieren test which additionally made refractive index gradient fine structure visible and detected scattering, axial symmetry and structural irregularity.

Aquatic vision and the modulation transfer properties of unlighted and diffusely lighted natural waters.

The modulation transfer function (MTF) of well-mixed unlighted and diffusely lighted samples of clear natural waters for path lengths up to 4 m was measured. The measuring conditions simulated the situation for horizontal aquatic vision. In unlighted water, the MTF decreased relatively slowly with increasing path length and spatial frequency up to 150 c/deg.