20/20 Vision

Visual acuity is usually measured with a Snellen chart. The Snellen chart displays letters of progressively smaller size. "Normal" vision is 20/20. This means that the test subject sees the same line of letters at 20 feet that a normal person sees at 20 feet. 20/40 vision means that the test subject see as 20 feet what a normal person sees at 40 feet. Another way of saying this is that a person with 20/40 vision has vision that is only half as good as normal - or, objects must be at half the normal distance for him to see them.
A person with 20/20 vision is able to read letters 1/10th as large as someone with 20/200 vision. However, 20/15 vision is better than 20/20. A person with 20/15 vision can see objects at 20 feet that a person with 20/20 vision can only see at 15 feet.


The Snellen Chart

Levels of Vision:

20/20 - Normal vision. U.S. Fighter pilot minimum. Required to read the stock quotes in the newspaper, or numbers in the telephone book.


20/40 - Able to pass Driver's License Test in all 50 States. Most printed material is at this level.


20/80 - Able to read alarm clock at 10 feet. News Headlines are this size.


20/200 - Legal blindness. Able to see traffic STOP sign letters.


To appreciate how the eye works, imagine yourself as a beam of light reflected from a 100-foot tree into the eye of a living person. You are the image of the tree, traveling at the speed of light and about to enter an obstacle course on your way to the brain of the observer.

Your first encounter is your passage through the clear convex cornea which bends (refracts) you and slows you down. It also shrinks you to a manageable size (a little larger than a nickel). Next you squeeze through a round, adjustable opening, the pupil, formed by a colorful membrane, the iris, which, if you are too bright, will reduce your intensity.

You now encounter a rather dense but transparent medium, the lens, which not only bends you even more, but unceremoniously turns you upside down. It then aims (focuses) you at the back of the eye, the retina, which you strike after passing through a clear, sticky, gel-like substance, the vitreous humor.


Your Prescription: The refractive error of the eye can be expressed in numeric terms. The power of the lenses necessary to correct your vision is measured in units called diopters (see below). The first number designates the amount of myopia (minus numbers) or hyperopia (plus numbers). The second number (if present), indicates the amount of astigmatism. PrescriptionThe third number shows the axis of the astigmatism. Depending on the sign of the second number, the axis indicates the orientation of the steepest or flattest meridian of the cornea. The fourth number is the "add", or the amount of magnification needed to give clear vision at near. If this number is present, bifocals have been prescribed to correct presbyopia.


What is a Diopter?

A diopter is a unit of measurement of the refractive error. It may be a negative number (myopia, nearsightedness), or a positive number (hyperopia, farsightedness). A -1.00 diopter myope is able to see objects at 1 meter clearly. A -2.00 diopter myope is able to see objects at a meter clearly. The greater the myopia, the strength of the lens in diopters needed to correct the refractive error, and the closer an object must be to be viewed clearly.


Mukilteo Vision Clinic - 8601 Mukilteo Speedway, Mukilteo, WA 98275 (425) 513-9186