The images an eye creates have to be well-focused for maximum information transfer capacity. It is, however, difficult to create well-focused color images because light of short wavelength (blue) is refracted more strongly than light of long wavelength (red). If light of intermediate wavelength (green) is in focus on the retina, blue light is focused in front of and red light behind the retina (longitudinal chromatic aberration). The resulting chromatic defocus is a particularly serious problem if the eye has a large pupil, such as in aquatic and nocturnal terrestrial species, because depth of focus is short in such eyes.
It was discovered in the African cichlid fish Astatotilapia (formerly Haplochromis) burtoni that the problem of chromatic defocus is solved by multifocal lenses (Kröger et al. 1999: Multifocal lenses compensate for chromatic defocus in vertebrate eyes. J Comp Physiol A 184, 361-369), a solution unique to biological optical systems. Multifocal optical systems are present in many freshwater and marine fishes, as well as lampreys, amphibians, reptiles, birds, and mammals.
With the multifocal principle as the common denominator, our research covers a wide range of topics, such as the function and structure of vertebrate crystalline lenses, constraints on eye design, the amazing biology of lens fiber cells, visual ecology, and the potential for technical application of biological optical designs.
A schlieren photograph shows the zones of a fish lens that focus different spectral ranges (colors).