You have probably heard the term 20/20 to describe healthy or “perfect” vision. Patients often ask what this means. 20/20 comes from a Snellen acuity chart which is a standard devised to measure someone’s eyesight relative to the average. It is what the average person sees clearly from 20 feet away. Few people have this so called “normal vision”, considering that 61% of Americans wear some form of corrective eyeglasses or contact lenses.
Individuals with poorer eyesight than the norm have a larger denominator. For example, instead of having 20/20 vision, you may have 20/40 vision. The first number represents the test distance in feet and the second number represents the size of the letters. A person with 20/40 vision can only see from 20 feet what a person with 20/20 can see from 40 feet. In Oklahoma, you must see at least 20/40 out of one eye to drive legally. This can be corrected or uncorrected vision.
Just because 20/20 is considered normal vision does not mean it is perfect. A small number of people see better than 20/20. The line of letters better than 20/20, or just below 20/20 on the acuity chart, is the 20/15 line. Recent technology in lens design and laser vision correction can, in rare instances, produced vision close to 20/10.
People often ask what is legally blind. Legal blindness is defined as best corrected visual acuity of 20/200 or less in the better eye; or a visual field limitation such that the widest diameter of the visual field, in the better eye, subtends an angle no greater than 20 degrees.
With respect to visual acuity for example, you might be 20/400 in one eye and 20/100 in the other with corrective lenses. This does not represent legal blindness. If the 20/100 eye drops to 20/200 then you are considered legally blind. Many people that wear contacts or glasses for distance vision often say “I’m legally blind without my glasses”. Being 20/400 in both eyes without your contacts or glasses is not legally blind as long as you are correctable to better than 20/200 in one eye. Of the 61% quoted above who require vision correction, most are correctable to vision better than 20/40, usually 20/20 to 20/30.
When respect to visual field limitations, this is referring to a patients side or peripheral vision. Patients who have glaucoma or have had a stroke for example, will often maintain good central vision however; permanently lose a large part of their peripheral or side vision. If a patient is limited to the widest diameter of their peripheral vision only being 20 degrees in their best eye, they are legally blind. Therefore, a glaucoma patient could be 20/20 however, have only 10 degrees of peripheral vision and be legally blind.
It is very important to know your best level of vision as it may apply to job requirements, various state and agency laws, as well as just being safe for job related activities and/or recreation. There are a few on-line computer vision tests that exist however, they are not reliable. You should only trust an eye care professional or a test administered by a trained individual, in the appropriate setting.
If you have questions about your vision or eye health, we encourage you to contact us in our Stillwater office at 405-372-1715 or Pawnee at 918-762-2573. We also invite you to visit our website at www.cockrelleyecare.com and like us on Facebook at Cockrell Eyecare Center!