How the Eyes Work

You may have seen the eyes being compared to a camera, and that is a good analogy. There are various eye structures that function like some of the camera parts:

  • The cornea (clear front dome) functions as the eye’s viewfinder
  • The lens is much like the camera lens
  • The pupil corresponds to a camera aperture, both changing their size to control how much light enters
  • The retina (back inside wall) is like a film, receiving images that need “developing” by the brain

There are no two human eyes exactly the same, microscopically speaking, but they all work on the same principles.

Vision Overview

Light is the first necessity for vision. It reflects off objects and arrives at the eye’s cornea. The cornea being transparent, the light travels through it, and through the equally clear fluid behind it (aqueous fluid).

The pupil widens in dim light and contracts in bright light, allowing the right amount of light into the eye to give clear vision. That light continues traveling inwards through the transparent lens and the second area of clear fluid behind the lens (vitreous gel).

Now the light has arrived at the retina. Retinal cells receive it and convert the light energy into neural energy. Each cell is supplied with a tiny nerve fiber. Each fiber carries this neural energy and they converge near the center of the retina. Here there is an opening where these fibers unite to form the optic nerve, which runs to the brain. The brain interprets the neural energy and supplies names for the images.

About the Retina

There are three types of light-sensitive (photoreceptor) cells in the retina:

  • About six million cone cells – providing sharp, colored vision in bright light; they are mostly clustered near the center of the retina;
  • About 120 million rod cells – providing vision in dim light, and they are mostly around the retinal periphery;
  • About 1.3 million ganglion cells – not involved in vision, but responsive to ambient light. They register the rising and setting of the sun and synchronize the body’s circadian rhythms to those lighting changes. They were discovered in the 1990s.

Each eye has a blind spot. That is near the retinal center at a place where there are no light-sensitive cells because the optic nerve leaves the eye at that spot. Nearby is the macula, where cones are clustered most densely. The center of the macula is the fovea, which gives us our central vision – direct and sharp. Peripheral vision is important too, but is indirect.

Accommodation and Focusing

If you picture a tree in the distance, light rays from the top branches and from the base of the trunk will be traveling almost parallel when they reach your eyes. Compare that to a tree just across the street – light from the top and base of this tree will arrive at your eyes more sharply angled.

This means that the eyes do not need to bend light rays from far objects very sharply because they are not sharply angled when they arrive. But light from nearby objects must be more sharply refracted (bent) to focus them on the retina.

If you have 20/20 vision and you glance from the distant tree to your watch, for example, the eyes automatically adapt to the different distances and give you clear images of both. The structure in the eye that does that is the lens. It changes its curvature from flatter for far distances to steeper for near distances and this is called Accommodation. You can read more on the Eye’s Lens and Cataract Surgery.

Before it reaches the lens, incoming light is first refracted (bent) by the cornea. But the cornea has a fixed curvature. The lens and cornea share the job of focusing light clearly on the retina, with the cornea doing most of the light refraction and the lens fine-tuning.

Common Vision Problems

If you are nearsighted (myopic), the fixed corneal curvature is bending light rays too sharply when they come from distant objects. This focuses them before they reach the retina. By the time that light does reach the retina, it has become scattered to some extent, giving you blurry images of distant objects. Stated another way, the corneal curvature is too steep for the length of the eyeball (front to back).

If you are farsighted (hyperopic), the fixed corneal curvature is not bending light rays from nearby objects sharply enough. Therefore they focus beyond the retina (or they would if they could pass through that tissue). This gives you blurry images of nearby objects. Stated another way, the corneal curvature is too flat for the length of the eyeball (front to back).

If you are astigmatic, the corneal shape is oval rather than round. The cornea thus has two curvatures: a flatter one on the longer axis and a steeper one on the shorter axis. Therefore light is focused more than once on the retina and you have blurriness for all distances.

These common vision problems can be corrected with procedures such as LASIK and PRK.

As a person ages, the lens may become clouded, a condition known as cataracts. It can be treated by cataract surgery, where the lens is replaced with an artificial lens called an intraocular lens (IOL). Please see our page on Technology for more IOL information.

To speak with an experienced ophthalmologist in your area and learn more about your vision, please contact us today.