Accommodation (vertebrate eye)

Your eyes must constantly adjust to see the world clearly. This process is called accommodation. It is how the eye changes its optical power to focus on objects at different distances. If you look at a far mountain and then a close book, your eyes must change how they bend light.

This bending of light is called refraction. To create a sharp image, the eye must bend light precisely onto the retina. The retina is the area at the back of the eye that receives these points of light.

How an eye focuses depends on where the animal lives. Land animals like birds and mammals usually change the shape of their lens. They use a group of muscles called the ciliary body to do this. In humans, the lens is held under tension by ligaments. When you look at something close, the ciliary muscle contracts. This relieves tension on the ligaments and lets the elastic lens become rounder and thicker.

A rounder lens has more refractive power for near objects. To see far away, the muscle relaxes and pulls the lens thinner. This makes the lens less curved so it can focus on distant things.

Scientists have studied these eye movements for a long time. Thomas Young first proposed that the lens changes shape in 1800. Later, in the mid-1800s, Hermann von Helmholtz and Thomas Henry Huxley refined this idea. They explained how the ciliary muscle works to round the lens for near vision. In 1992, Ronald Schachar proposed a different model. His theory suggested that different muscles work together to change the lens shape more subtly.

This helps the eye correct for distortions while it focuses.

Different animals use different tools to achieve focus. Fish and amphibians live in water, so they have different needs. Because the difference in light bending between water and their eyes is small, they must use internal structures more. Instead of changing the lens shape, they often move a rigid lens closer to the retina.

In humans, the eye can change its focus very quickly. In bright light, this can happen in about 224 milliseconds. The human lens also has different layers. The center layers have a refractive index of about 1.406, while the outer layers are about 1.386.

As we get older, our ability to focus changes. This is a natural part of growing up. The amplitude of accommodation, or the range of clear vision, declines with age. By the time people reach age 45 to 50, many notice it is harder to see close objects. This is called presbyopia. By age 70, the ability to focus has decreased even more.

This is why many people eventually need glasses for reading. It is just the way the eye's muscles and lens change over time.