Originally posted by Leaf Fan So far so good. God blessed me with better than perfect eyesight (40/20) and hopefully that lasts as long as possible.
I'm confused as to what your measurement of 40/20 really means.
If using the Snellen chart the first number will always be 20 as it represents the test distance (in feet) the subject is from the eye chart. That shouldn't change in the Snellen test other than using metric measurements and then the number would be 6 instead of 20.
The second number represents the distance that the average eye can see the letters on a certain line of the eye chart. So, 20/20 means that the eye being tested can read a certain size letter when it is 20 feet away. If a person sees 20/40, at 20 feet from the chart that person can read letters that a person with 20/20 vision could read from 40 feet away. The 20/40 letters are twice the size of 20/20 letters; however, it does not mean 50% vision since 20/20 sounds like it is one half of 20/40. If 20/20 is considered 100% visual effiency, 20/40 visual acuity is 85% efficient.
(This is the way vision acuity was explained to me by my opthamologist.
The text above is quoted from
this website. )
A recent eye test at my Dr.'s office (for an upgrade in my driving license) showed one eye to be 20/10, the other 20/15 and combined she noted my vision acuity as 20/15; slightly better than the infamous 20/20 most people seem to benchmark.