Fig. 1. Schematic illustration of the function of a colour-corrected multifocal fish lens. The spherical lens has a number of discrete zones, three in this example, of different focal lengths for monochromatic light of intermediate wavelength (green). Because of colour dispersion, the lens refracts light of short wavelength (blue) more strongly than green light, such that the zone of the lens having too long a focal length for green light focuses blue light on the retina. Accordingly, the zone of the lens having too short a focal length for green light focuses light of long wavelength (red) on the retina. By this mechanism, a sharp colour image is created by a single lens. That image, however, is contaminated by defocused light having passed through `wrong' zones of the lens (e.g. the peripheral and intermediate zones for green light).