You have probably heard the term 20/20 to describe normal vision. Patients often ask what this means. 20/20 comes from a Snellen acuity chart which is a standard devised to measure a person’s eyesight relative to the average. It is what a 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 or numerator represents the test distance in feet and the second number or denominator 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 meaning with or without glasses or contact lens.
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 eye chart, is the 20/15 line. Recent technology in spectacle lens designs and laser vision correction can produced vision of 20/15 and in some individuals 20/10.
People often ask what is legally blind. Two different parameters determine legal blindness. One is visual acuity as described above and the other is visual field or field of vision. The visual field refers to the total area in which objects can be seen while looking centrally and is measured in degrees. For example, while looking straight ahead a person may be able to see with their right eye objects 90 degrees to the right, 50 degrees to the left, 50 degrees superiorly, and 60 degrees inferiorly. We also refer to this as peripheral or side vision.
Legal blindness is having 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, is no greater than 20 degrees.
For example, with respect to visual acuity, you might be 20/400 in one eye and 20/100 in the other eye with corrective lenses. This does not represent legal blindness. If the 20/100 eye becomes reduced 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 if 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.
With respect to visual field limitations, patients who have advanced 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. If you have questions about your vision or eye health, we encourage you to contact us in Stillwater at 405-372-1715. We also invite you to visit our website at www.cockrelleyecare.com and message us on Instagram and Facebook at Cockrell Eyecare Center!