Accommodation is the process by which the vertebrate

Vertebrate

Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

 eye

Eye

Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...

 changes optical power

Optical power

Optical power is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device. The dioptre is the most common unit of measurement of optical power...

 to maintain a clear image (focus

Focus (optics)

In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...

) on an object as its distance changes.

Accommodation acts like a reflex, but can also be consciously controlled. Mammals, birds and reptiles vary the optical power by changing the form of the elastic lens using the ciliary body

Ciliary body

The ciliary body is the circumferential tissue inside the eye composed of the ciliary muscle and ciliary processes. It is triangular in horizontal section and is coated by a double layer, the ciliary epithelium. This epithelium produces the aqueous humor. The inner layer is transparent and covers...

 (in humans up to 15 diopters). Fish and amphibians vary the power by changing the distance between a rigid lens and the retina with muscles.

The young human eye can change focus from distance to 7 cm from the eye in 350 milliseconds.