Accommodation (vertebrate eye)

Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image or focus on an object as its distance varies. In this, distances vary for individuals from the far point—the maximum distance from the eye for which a clear image of an object can be seen, to the near point—the minimum distance for a clear image. Accommodation usually acts like a reflex, including part of the accommodation-convergence reflex, but it can also be consciously controlled.

The main ways animals may change focus are:

Focusing the light scattered by objects in a three dimensional environment into a two dimensional collection of individual bright points of light requires the light to be bent. To get a good image of these points of light on a defined area requires a precise systematic bending of light called refraction. The real image formed from millions of these points of light is what animals see using their retinas. Very even systematic curvature of parts of the cornea and lens produces this systematic bending of light onto the retina.

Due to the nature of optics the focused image on the retina is always inverted relative to the object. Different animals live in different environments having different refractive indices involving water, air and often both. The eyes are therefore required to bend light by different amounts leading to different mechanisms of focus being used in different environments. The air/cornea interface involves a larger difference in refractive index than hydrated structures within the eye. As a result, animals living in air have most of the bending of light achieved at the air/cornea interface with the lens being involved in finer focus of the image.

Generally mammals, birds and reptiles living in air vary their eyes' optical power by subtly and precisely changing the shape of the elastic lens using the ciliary body.

The small difference in refractive index between water and the hydrated cornea means fish and amphibians need to bend the light more using the internal structures of the eye. Therefore, eyes evolved in water have a mechanism involving changing the distance between a rigid rounder more refractive lens and the retina using less uniform muscles rather than subtly changing the shape of the lens itself using circularly arranged muscles.

Varying forms of direct experimental proof outlined in this article show that most non-aquatic vertebrates achieve focus, at least in part, by changing the shapes of their lenses.

What is less well understood is how the subtle, precise and very quick changes in lens shape are made. Direct experimental proof of any lens model is necessarily difficult as the vertebrate lens is transparent and only functions well in the living animal. When considering vertebrates, aspects of all models may play varying roles in lens focus. The models can be broadly divided into two camps: those models that stress the importance of external forces acting on a more passively elastic lens and other models that include forces that may be generated by the lens internally.