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How the Eyes Work

Jennifer Kimberley

The eye is often compared to a camera, and for good reasons. Several eye structures function in a similar way to some of an analog camera's parts:

  • The eye's cornea (front clear part) is like a camera viewfinder
  • The eye's lens corresponds to a camera lens
  • The eye's pupil changes its size like a camera aperture
  • The eye's retina functions like a camera film

Each human eye is unique. When you have a Custom LASIK procedure, your two eyes are diagnosed separately and the LASIK system will express their contours on the monitor as two colored 3D maps. Your treatment uses this data to give you a unique treatment for each eye, and nobody else has ever had or will ever have your exact two treatments.

But the principles in how a human eye works are the same in all cases.

Summary of How We See
Light bounces off objects and enters the eye through the cornea. It then travels through:

  • The clear fluid (aqueous humor) behind the cornea
  • The pupil (opening in the colored iris)
  • The crystalline lens suspended behind the pupil
  • The clear gel behind the lens (vitreous humor)

It has now arrived at the eye's inside back surface called the retina. The retina is filled with light-sensitive cells called cones and rods.

  • Cones give us color vision in bright light and are densely clustered in the central area of the retina. This area is called the macula and its pinpoint center is the fovea. This area gives us our central vision - the clear direct vision that we use for reading, computer work, sewing etc.
  • Rods give us our night vision and are mostly clustered around the retinal periphery.

The Optic Nerve
Each eye has an optic nerve which connects the retina to the brain. It leaves the eye near the retina’s center from an area called the optic disc. This is also the eye’s blind spot because there are no rods or cones here. The optic nerve is large, made up of 1.2 million individual fibers.

After the rods and cones convert image information to electrical energy, the nerve fibers carry it to the brain. They travel across the retina to the optic disc, converge to form the optic nerve, and travel together inside a nerve sheath to the brain’s vision center at the back of the head.

The brain then interprets this electrical energy and finds words for those images.

How is Clear Focus Achieved?
When light enters the eye, it is traveling from some object which has been illuminated by this light. It bounces off the object and if there is an eye in the vicinity some of it will enter that eye. Depending on the distance between the object and the eye, the angle of the light rays is different.

  • Light from far objects enters the eye at a fairly flat angle, in almost parallel rays. For example, the light from the top of a distant tree enters almost parallel to the light from the tree's lowest branch.
  • Light from near objects has less distance to travel and enters at sharper angles. So if you are looking at a tree close to you, the light from the top is entering the eye at a sharp downward slant, whereas the light from the lowest branch may enter at almost a flat slant.

It is the eye's job to change those diverse angles such that they will converge on the retina and create a clear image. It will be an upside down image.

All lenses (transparent curved structures) refract (bend) light that passes through them. Each eye has two lenses which bend incoming light:

  • The cornea
  • The crystalline lens

As we switch focus from near to far objects, the cornea refracts light in a fixed way because it is unable to change its curvature. However the lens changes its curvature depending on how far away an object is. This is known as accommodation - the eye's ability to accommodate itself to the variety of angles at which light enters it.

Refractive Errors
Refractive errors are mistakes in the bending of light which give blurry vision. A steeply curved cornea refracts light more sharply than a flatter cornea.

  • If you are nearsighted (myopic), your eyes are refracting light from distant objects too much so it is focusing before it gets to the retina. Nearby objects need the strong refraction to focus on the retina so your near vision is clear but your far vision is blurry. A myopic eye has a cornea which is too steep for the eye's length front-to-back.
  • If you are farsighted (hyperopic), your eyes are not refracting light from near objects enough. So the light is focusing behind the retina (or would be if it could travel through that dense tissue). But the light from far objects is refracted enough and your far vision is clear while your near vision is blurry. A hyperopic eye has a cornea which is too flat for the eye's length front-to-back.
  • If you are astigmatic, your cornea is not round. It has a slightly oval shape like a football. This shape has two curvatures: a steeper one on its shorter side and a flatter one on its longer side. So it refracts light at two angles and gives blurry vision at all distances.

Custom LASIK and its variations correct these three refractive errors.

If You Are Presbyopic
A presbyopic eye may have an excellent relationship between its corneal curvature and its length. It may be a 20/20 eye. But the lens is losing its accommodative ability. It is becoming fixed in a flatter curvature and cannot steepen itself enough to give you clear close-up vision. The cornea cannot compensate because it cannot change its curvature.
Presbyopia can be corrected by monovision, where one eye is left as it is for far vision and the other is corrected for near vision. This can be done using contact lenses, LASIK Monovision, or Conductive Keratoplasty (CK).

Presbyopia can also be corrected by replacing the eye’s natural lens with an intraocular lens (IOL) such as Crystalens®.

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