Visual acuity is quite simply the ability to see the finest details of an object, whether it is a tree on a far-away mountain or the intricate pattern of a butterfly’s wing. The most common way we speak of visual acuity though says nothing about our visual performance up close.
The most over-used phrase that relates to visual acuity is 20/20 vision. If you have “20/20” vision it just means that one or both of your eyes can see the smallest detail of something that is 20 feet away, the same detail that the average “normal” person can see. it is just a ratio of the test distance to the letter size.
20/40 (test distance/letter size) vision on the other hand means that if you are twenty feet from a small letter, it has to be twice as big for you to see it clearly (20 x 2 = 40 in the ratio). That eye’s vision is therefore poorer than average.
The funny thing is, the average human being should actually be able to see a slightly smaller letter at 20 feet away, or the equivalent of 20/15 vision (notice the “15” is the letter size, and 15 is less than 20). It’s not as catchy a phrase though is it?
For normal healthy human eyes, this ratio should be the same when viewing objects that are far away as they are when looking at up-close objects, like words on a page. Many factors can influence your visual acuity, such as: not wearing the proper glasses, poor focusing ability, a poor tear film on the surface of the eye from dryness, damage to the clear surface of the eye (the cornea), cataracts (lens of the eye), macular degeneration (the retina), and developmental neurological abnormalities such as amblyopia (in the visual cortex of the brain).
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