Magnific's Inc. - Cell Phone Magnifying Lens Accessories - Your vision is our vision

Measuring Vision

Vision standards are necessary to enable comparisons of people’s visual capabilities. The standard that is widely used today is the eye chart based on the work of Dr. Hermann Snellen. The Dutch ophthalmologist designed a system for describing human vision back in 1862.  Dr. Snellen found that certain letters of the alphabet, when of a certain size, could be seen easily at a particular distance by the average, normal eye. This relationship—size of letters viewed from a specific distance—has become a standard way for recording visual acuity.

The Snellen eye chart is composed of a series of letters, the largest at the top of the chart and then getting gradually smaller to the bottom of the chart. For small children and others who can’t read the alphabet, eye charts are composed of objects other than letters. Examples are pictures of broken circles with a missing segment facing in different directions, or small pictures of common objects.

The Snellen fraction-like numbers have nothing to do with the power of your eyeglasses. It cannot be used for ordering a pair of eyeglasses or contact lenses to correct your vision.

What Does 20/20 Mean?

When visual acuity is measured, each eye is always checked alone while the other eye is covered. In the term, 20/20, the first number represents 20 feet—the distance between the eye being tested and the eye chart. The second number represents the distance that an average eye can see the letters on a particular line of the chart. Thus, 20/20 indicates that the eye being tested can read a specific “normal” size letter when t is 20 feet away.

You may have 20/20 vision without corrective glasses or only when wearing the proper correction, so that fact needs to be noted. It is a person’s corrected vision that is most important in determining best vision capability.

20/20 vision describes visual acuity only. It does not guarantee the absence of an eye disease or any other type of eye problem. However, it has proven to be a surprisingly reliable indicator that the eyes are normal.

What does 20/40 Mean?

If your vision is 20/40, this means that at 20 feet from the chart you can only read letters that a person with 20/20 vision could read from 40 feet away. (The letters are twice the size of 20/20 letters.) Although 20/40 sounds like one-half of 20/20, it does not mean 50% less vision. Based on 20/20 as a 100% visual efficiency standard, 20/40 vision is actually 85% efficient.

What is Legal Blindness?

In all Federal and State visual codes, individuals whose best corrected acuity is 20/200 or poorer are classified as “legally” blind. Those levels of poor acuity qualify that individual for some defined social services.  If your vision is correctable to better than 20/200, you are not considered legally blind no matter how poor your uncorrected vision may be.