Eye Care Resources

About The Eye and Eye Care

How is vision measured?

Having 20/20 vision means seeing at 20 feet what a person with normal vision sees at 20 feet. A person who has 20/40 vision can see at 20 feet what the person with normal vision sees at 40 feet. And so on.

The eye does not actually “see” objects. Instead, it sees the light that objects reflect. To see clearly, light striking the eye must be bent or “refracted” through the cornea–the clear window at the front of the eye that provides most of the focusing power. Light then travels through the lens, where it is fine-tuned to focus properly on the nerve layer that lines the back of the eye–the retina–and sent to the brain via the optic nerve. The retina acts like the film in a camera, and clear vision is achieved only if light from an object is precisely focused on it. If not, the image you see is blurred. This is called a refractive error.

Refractive errors usually occur in otherwise healthy eyes. They are caused mostly by an imperfectly shaped eyeball, cornea or lens. There are four basic types of errors:

Myopia or nearsightedness— Close objects appear sharp but those in the distance are blurred. The eyeball is longer than normal from front to back, so images focus in front of the retina instead of on it.

Hyperopia or farsightedness— Distant objects can be seen clearly but objects up close are blurred. The eyeball is shorter than normal, so images focus behind the retina.

Astigmatism–Objects are blurred at any distance. The cornea, lens, or both are shaped so that images aren’t focused sharply on the retina.

Presbyopia or aging eye–The eye loses its ability to change focus due to the natural aging process. This usually occurs between ages 40 and 50.