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).