Losing focus: How lens position and viewing angle affect the function of multifocal lenses in fishes

Light rays of different wavelengths are focused at different distances when they pass through a lens (longitudinal chromatic aberration; LCA). For animals with colour vision this can pose a serious problem, because in order to perceive a sharp image the rays must be focused at the shallow plane of the photoreceptor outer segments in the retina. A variety of fish and tetrapods have been found to possess multifocal lenses, which correct for LCA by assigning concentric zones to correctly focus specific wavelengths. Each zone receives light from a specific beam entrance position (BEP) – the lateral distance between incoming light and the centre of the lens. Any occlusion of incoming light at specific BEPs changes the composition of the wavelengths that are correctly focused on the retina. Here, we calculated the effect of: lens position relative to the plane of the iris; and light entering the eye at oblique angles on how much of the lens was involved in focusing the image on the retina (measured as the availability of BEPs). We used rotational photography of fish eyes and mathematical modelling, to quantify the degree of lens occlusion. We found that at most lens positions and viewing angles there was a decrease of BEP availability, and in some cases complete absence of some BEPs. Given the implications of these effects on image quality, we postulate that three morphological features (aphakic spaces, curvature of the iris and intraretinal variability in spectral sensitivity) may, in part, be adaptations to mitigate the loss of spectral image quality in the periphery of the eyes of fishes.