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Fig. 2. The functional significance of the slit pupil in combination with a
multifocal lens. In the fully dilated state of the pupil (A), all zones of the
lens (shown in the colours they are focusing) can be used. A concentrically
constricting iris (B) would cover the outer zone of the lens such that a
spectral range (blue in this example) could not be focused on the retina. By
contrast, all lens zones can be used if the pupil constricts to a slit
(C).