MULTIFOCAL SPHERICAL FISH LENSES

Vision is an important source of information for many animals. The lens plays a central role in the vi-
sual pathway and hence the ecology of these animals. Fish have spherically shaped crystalline lenses that
contain a gradient of refractive index. Like all refracting elements, lens performance depends on the wave-
length of the refracted light. This wavelength dependent focusing leads to chromatic aberration where only
one wavelength interval is being focused correctly. Fish lenses compensate for this by dividing the lens into
multiple concentric zones each responsible for focusing a particular wavelength interval. In this thesis I
explore the optical and visual benefits these multifocal lenses offer. In order to minimize light scattering by
the lens, all cells except the lens periphery are devoid of nuclei and cell organelles. Interestingly, the refrac-
tive index of this outer most zone is constant, maintaining a physiologically stable environment for these
cells to function (Paper I). It has been shown that light or dark-adapting fish change their lenses’ properties
within hours while fish reared under differently colored artificial light regimes do so after several months. I
explored the effects of the light regimes found in the Mediterranean and the Red Sea on lenses of the rivu-
lated rabbitfish, Siganus rivulatus. Such natural spectral differences affected the mean focal length of the
lenses making the Mediterranean fish eyes more light sensitive than those found in the Red Sea population
(Paper II). This coincides well with the darker and tinted waters found in the Mediterranean. Dispersion
describes the wavelength dependency refractive media have. I therefore described a dispersion model that
fitted measured refractive indices of various ocular media as well as the longitudinal chromatic aberration
determined by laser scanning of fish lenses (Paper III). Relying on the results of the previous papers (Paper
I & III), I compared multifocal fish lenses to a simplified monofocal lens. The multifocal lenses showed
to be superior to monofocal ones. While monofocal lenses perfectly focus one wavelength, the short focal
length in combination with the chromatic aberration these fish lenses have degrades the image at all other
wavelengths. This results in an image relatively rich in spatial information (i.e. high contrast and sharp
edges) but no spectral integrity (wrong hues and bland colors). Multifocal lenses offer an optimal trade off
between spatial and spectral information (Paper IV).