What is 20 20

Hopefully your eye exam will find your vision to be 20/20, but what does that mean and how does it compare to everyone else out there?

In the US, the ability to see visual detail (visual acuity) is described in a fraction. The first number indicates the distance to the visual target. This is 20 feet for the standard in the US and 6 meters for most other countries. Most exams rooms are not 20'. This is why it's common to see mirrors which lengthen the target distance in smaller exam rooms. The second number is the size of the target letter. For 20/20, the target letter is 8.87mm (0.35") tall by 8.87mm wide. Therefore, 20/20 is the ability to identify an 8.87mm character at 20'.

Of course some letters are easier to identify than others. Letters with straight lines, such as an "E" are more readily recognized than circular letters "O". Straight lines provide more clues to the brain as it is putting the visual picture together. Some patients may notice that letters change what they appear to be the longer they look at the chart. This is due to visual closure which occurs when the brain is attempting to discern detail at its threshold. Through visual closure the brain takes incomplete visual information and attempts to fill-in the blanks.

The distance of 20 feet for test distance is selected for "optical infinity" or the distance at which the eye doesn't need to accommodate to focus. When the eye accommodates it shifts it's power more nearsighted than it is when focusing in the distance. The 20' test distance prevents excessive minus or nearsighted power from being in the prescription. Excess minus power can cause headaches and discomfort when reading or on the computer.

Roughly 90% of eyes are capable of 20/20 vision with glasses or contacts. Also, 60 to 70% of eyes are capable of 20/15 vision. The best visual acuity ever recorded was 20/6.7. Although this patient was constantly bothered with slight imperfections due to floaters and debris inside the eye and on the front surface of the eye. In studies of patients with autism, visual acuities of 20/7 have been noted. In theory, the human (bionic) eye is capable of 20/5 vision. Most professional baseball players possess 20/12 or better visual acuity.

There are times patients aren't able to read the 20/20 line. This can be due to a lazy or weaker eye (strabismus, amblyopia) or it can be due to a health problem affecting the eye and/or brain. Many patients will say, "I'm blind without my glasses!" This is okay to say in the exam room, but may be a bit insensitive if out in public. For anyone over 2 diopters of nearsightedness the "big E" (20/400) can be difficult to see without correction (glasses, contacts). There are individuals in the world that truly are blind and have difficulty seeing the "big E" even with correction.

Blindness and low vision are broken down into several categories:
Mild visual impairment = 20/30 to 20/60
Moderate visual impairment = 20/70 to 20/160
Severe visual impairment = 20/200 to 20/400
Profound visual impairment = 20/500 to 20/1000
Near total visual impairment = worse than 20/1000
Total blindness = NLP (no light perception)

Legal blindness is 20/200 or worse and visual field of 20 degrees or less

For testing beyond 20/400 special charts or modification of test distance are required. If letters cannot be read visual acuity is measured in ability to count fingers, see hand motion, or perceive presence of light.

In summary
20/20 is the ability to see 1.75mm of detail at 20 feet.
Most people can see 20/20 if the eye and brain are healthy and have developed normally.
Try to avoid being insensitive to those who truly are blind.
Many people are legally blind without correction
There is a big difference between see with glasses or contacts and visual impairment.