Vision Best answer on the web

Author: admin     07 30th, 2010 in xn--00t609f.com     edit

  • How do you figure out what a person's uncorrected visual acuity is, expressed as 20/x, if the person's corrected visual acuity is 20/20, and you know the person's prescription? For example, if my eyes are correctible to 20/20 with -8.0 diopter lenses (that is, lenses with focal length -12.5 cm), how well do I see without glasses? 20/1000 or what? There are online references that say there is no exact relationship. There must be at least an approximate relationship--what is it?


  • Hello, racecar.

    It's true that the 20/something numbers associated with the Snellen visual acuity chart do not have precise equivalents in diopters. I squabbled with an ophthalmologist about this many years ago. I asked him for my exact "20/something" number, and he gave me a bunch of rigmarole which basically translated to "I dunno." As it turns out, "I dunno" was the right answer; visual acuity is affected by several factors in addition to the curvature of the eye's lens. I finally wrung a guesstimate out of an optometrist, who said that my correction of -7.75 diopters was in the vicinity of 20/1000. He may have pulled this out of his navel, of course, but it seems to jibe with what I've learned elsewhere.
    I have found a chart that gives some rough approximations of "20/something" values for myopic (nearsighted) patients:
    "Eyesight is commonly evaluated by reading a Snellen eye chart from a distance of 20 feet - the distance beyond which the lens does not need to adjust for distance. (This is the equivalent of the infinity setting on a camera.) Variances from the 'ideal' 20/20 can be caused by myopia, astigmatism and other variables including the lighting in the room. 20/something provides a descriptive approximation of visual acuity.
    The power of a lens placed in front of the eye to correct for myopia is measured in terms of diopters. Diopters are a prescriptive measure of myopia. If myopia is the only variable, the following approximate relationships exist between measurements in terms of diopters and in terms of 20/something:
    In Diopters 20/something
    -------------------------------
    -0.50 20/25 to 20/30
    -1.00 20/30 to 20/50
    -3.00 20/300
    -4.00 20/400
    -5.00 20/600
    -6.00 20/800 "

    LPF: Diopters and 20/Something
    http://www.lpf.com/source/rk/20something.html

    My Google search strategy:

    Google Web Search: 20/100 20/200 diopters
    ://www.google.com/search?hl=en&q=20%2F100+20%2F200+diopters

    I hope this is precisely what you need. If it is not, please request clarification, and I'll resume the quest.
    Best regards,
    pinkfreud









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