Normal Vision


We “see” things every day, from the moment we get up in the morning until we go to sleep at night. Sight is our primary sense for maneuvering around in our environment. It keeps us safe and allows us to find wherever we wish to be. Through sight, we are able to appreciate the graceful human form, a kids’ crayon drawing, a fine oil painting, swirling computer graphics, gorgeous sunsets, blooming gardens, rainbows and so much more. We rely on mirrors to make ourselves presentable, signs to give us direction, and sparkling gemstones to show affection.

Chart showing how an eye sees

But, did you ever stop to think that when we see any of these things, we are not directly connected to them? And that we are, in fact, only seeing light – light that bounces off objects far or near and into our eyes. Light is all our eyes can really see.

How it works

For people with normal vision, the following sequence takes place:

  1. Images start out as light rays bouncing off objects and reflected into your eye. This is why you cannot see in absolute darkness.
  2. At an amazing speed, the reflected light enters the front of the eye through a clear, dome-shaped surface called the cornea. The rounded shape of cornea bends the light towards the center of the eye. This bending is called refraction.
  3. From the cornea, light is bent through the dark central opening in the middle of your eye called the pupil. The size of the pupil limits the amount of light passing through and is controlled by the pigmented (colored) muscular part of your eye called the iris. The iris contracts or expands in response to the brightness of available light.
  4. Past your pupil, the light goes through a transparent, double-convex structure – like this: () called the crystalline lens. This lens further bends (refracts) the light rays to form a sharper focal point and is controlled by tiny muscles that make the lens fatter or thinner depending on the distance of the object you are viewing. This flexing of the lens is called accommodation and allows you to change your focus from near objects to far objects and back again. As you grow older this flexing ability diminishes, which is why you see people over the age of forty using glasses to read.
  5. The more highly focused light passes through a clear, jelly-like substance that fills the center of your eye called the vitreous humor.
  6. As its final destination, the light reaches the incredibly sensitive, nerve-dense area on the back of your eye, called the retina.
  7. The nerves of the retina transform light rays into electric signals.
  8. Bundled together at the optic nerve, all of these signals are sent to your brain.
  9. A special area in your brain, called the visual cortex, translates the signals into images called vision. Thus, your brain is the actual organ of sight!

Limiting factors

In normal vision, 20/20 vision light focuses on the center of the retina.

Where the light is focused on the retina is determined by the length and shape of the eyeball:

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  • If the eyeball is too long in length, then the focused image will be in front of the retina. This condition is called myopia (or nearsightedness).
  • If the eyeball is too short in length, the focused image will be behind, or beyond, the retina. This condition is called hyperopia (or farsightedness).
  • If the eyeball is out of round, the focused image will be in more than one place on retina. This condition is called astigmatism.
  • In the case of presbyopia, the crystalline lens loses its flexibility. The ability to focus is limited to one distance.

When the brain perceives out-of-focus images the result is called a refractive error (that is to say, when the light came through the eye there was an “error” made in bending it correctly).

20/20 vision

Vision or visual acuity is primarily tested using a Snellen Eye Chart. By examining lots of people, eye doctors have decided what is normal for human being to be able to see when standing 20 feet away from the eye chart.

If you have 20/20 vision, it means that when you stand 20 feet away from the chart you can see what the majority of people can see at that same distance.

If you have 20/40 vision, it means that if a normal person were standing 40 feet away from the chart, and you were standing only 20 feet away from the chart, you and the normal person would see the chart with the same level of clarity.

20/100 vision means that when you stand 20 feet from the chart you can only see what a normal person sees when standing 100 feet away.

20/200 vision and over is the qualification of legal blindness in the United States.

Better than 20/20

You can also have vision that is better than normal. If a person has 20/15 vision they would be seeing at 20 feet what a normal person sees when standing 15 feet away from the chart.

Hawks, owls and other birds of prey have much more acute vision than humans. Although a hawk has a much smaller eye than a human being, it has more cones packed into that space and two foveas. A hawk’s vision is eight times more acute than a human’s: a hawk might have visual acuity of 20/2.

However, with today’s Wavefront technology, it is possible for human eyes to achieve 20/15 or better.

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