There are several structures in your eye responsible for forming the images that you see. The cornea and crystalline lens focus light by bending it as it enters the eye, so the light rays form an image on the retina. The cornea is the clear outermost layer of your eye. The crystalline lens inside the eye also bends light rays as they pass through the eye. The retina is the part of the eye that transmits information to the brain, allowing you to “see” images. The fovea is the central point on the retina that produces the sharpest vision.

The eye focuses incoming light by bending all the light rays to meet at a single point on the retina. When all of the light rays meet on the retina, it gives you a clear, sharp image of the object at which you are looking. However, if the light focuses behind the retina or in front of the retina, the image you see will be blurred. Depending on where the image focuses, you may be nearsighted or farsighted. Because the cornea has the greatest bending (focusing) power, it is the cornea’s shape that determines the eye’s focusing power. In normal eyes, the cornea is able to bend light rays to focus on your retina.

In a normal eye, light is focused on the retina.

Measuring your vision

A person with normal or average vision is considered 20/20, which refers to what he/she should be able to see when standing 20 feet away from an eye chart. If you have 20/25 vision, it means that when you stand 20 feet away from the chart you can see what a normal person can see when standing 25 feet from the chart. That is, if a person with 20/20 vision is standing 25 feet away from the chart and you are standing only 20 feet away from it, you and the normal person can see the same detail. Similarly, if you have 20/40 vision, it means that when you stand 20 feet away from the chart you can see what a person with 20/20 vision can see when standing 40 feet from the chart (see diagram below). In the United States, 20/200 is the cutoff for legal blindness.

What is 20/20 vision?

At 20 feet, the person with 20/20 vision sees the stop sign clearly.
At 20 feet, the person with 20/40 vision sees the stop sign as if they were 40 feet away.