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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).
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