The Human Eye and How Vision Works

How Your Vision is Measured

Most people who have had an eye exam that includes a test to measure visual acuity, clarity or sharpness of vision, recognize the simple notation 20/20 as meaning "normal vision." What do those numbers mean? Let's say your vision is 20/40. That means you can see at twenty feet what a person with normal vision can see at forty feet. Your measure of visual acuity is deter mined by using the Snellen chart, that familiar eye chart with progressively smaller letters on each line. Although it is considered an accurate vision test, its results are sometimes affected by such variables as squinting, guessing at the letters, and room light.

So, numbers such as 20/20 or 20/40 describe your visual acuity but do not measure your refractive error-how accurately your eye bends, or refracts, light. When an eye doctor measures your refractive error, what you end up with is your eyeglass prescription. Finding an eye doctor whose measurements are impeccable is crucial, not just for your eyeglass prescription but also, as you will learn later, for laser vision correction.

Understanding Your Eyeglass Prescription »