Figure. 6:

This illustration offers the basic rationale for using aberrations, or multifocality, to extend the eye’s depth of field. In both cases, the eye has the same amount of negative spherical aberration. Upper figure: Light from a distance object is refracted by the optics, and the marginal rays (bold and colored green) focus on the retina, while the paraxial rays focus in front. Lower figure: For a near object, the paraxial rays focus on the retina and the marginal rays focus beyond the retina. Constriction of the pupil, which is generally associated with convergence on near objects further augments the effect by blocking some of the non-focusing rays. The vignette rays are shown as dashed in the lower figure. Although the pupil constriction mechanism may be effective for near work, keep in mind that the same pupil constriction would also render the eye myopic when the pupil constricts in bright lights.