P were varied. When the ciliary muscles contract, they loosen the ciliary fibers which are attached to the envelope of the crystalline lens . Because the lens is pliable, it relaxes into a more curves shape, increasing it's refractive power to accommodate for closer viewing. The iris serves as the aperture stop for the eye, closing to about 2mm in diameter in bright light and opening to a maximum of about 8mm in dim light.
Many animals can see clearly both in water and on land.
Some have extraordinary accommodation ranges, and others have developed other strategies. Cormorants and dippers can vary the refractive power of their lenses by 40-50 diopters, compared to about 16 diopters for an average adolescent human. The increased accommodation is due largely to highly developed sphincter muscles which vary the curvature of the front of the lens. Turtles and otters also have very strong sphincter muscles. Variations in lens geometries are used in various species of birds and fish.