better than 20/20

Discussion in 'Optometry Archives' started by a06812, May 21, 2007.

  1. a06812

    a06812 Guest

    Hi all

    Can someone explain the difference between being able to see better
    than 20/20 (or 6/6) say for example 20/15, and hyperopia?

    For example, an optometrist can correct a myope to 20/15 sometimes.
    How does he know that he is not overcorrecting?

    a06812, May 21, 2007
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  2. a06812

    Jan Guest

    Richard, vision acuity is just an agreement about a standard notation.
    It only describes what you can see with or without a correction.

    All over the world it is used, some countries with 20/20 (America), 6/6
    for the old fashioned Englishmen and for the very modern Europeans it is
    written as 1.0

    You can have a better vision acuity (i.e. 20/15 or 6/5 or 1.2) or a
    worser (20/40 or 6/3 or 0.5)

    An emmetropic eye (no correction needed) can also be valued.

    Overcorrecting a myope never results in a better vision acuity, even so
    does undercorrecting a hyperopic.

    Vice versa, undercorrecting a myope results in a worser vision acuity
    and overcorrecting in a hyperopic does the same worse thing.

    Jan (normally Dutch spoken)
    Jan, May 21, 2007
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  3. acuity of 6/6 is a sort of "normal" clinical benchmark. it is not the
    best acuity a human can achieve, which is around 6/4.5.
    hyperopia describes a refractive error in which light tends to focus
    behind the retina. many hyperopes see 6/6 or better with no glasses/
    the persons best acuity (6/6, 6/4.5 etc.) would be measured with their
    refractive error corrected (glasses, contacts, laser surgery eg.).
    Over-correcting will not produce better acuity. if a myope sees 20/15
    it does not mean he/she is overcorrected. Some overcorrection can be
    tolerated and still show 20/15 though.

    as for knowing if someone is overcorrected there are a few ways, one
    of which is testing with a red=green filter over the snellen chart
    which causes one side of the chart to be seen on a green background
    and the other side in red. (overcorrected myopes see letters more
    clearly on the green side, opposite for overcorrected hyperopes).
    michael toulch, May 21, 2007
  4. a06812

    Jan Guest

    michael toulch schreef:
    Interesting, I almost never use the R/G test, not reliable when people
    still are able to accommodate TMHO.

    The vision acuity is my leading issue when refracting.

    Jan (normally Dutch spoken)
    Jan, May 21, 2007
  5. a06812

    a06812 Guest

    Thanks everyone for your replies. I found it them interesting. The
    binocular effect was pretty much what I was thinking of - that if you
    were an over corrected myope you would be like a hyperope, therefore
    able to see things further away clearly.

    I like Mike's idea for checking the final prescription. I have a
    history of being given the "wrong" prescription, although it's often
    been a case of -0.25 too much sphere but -0.25 too little cylinder
    along with an axis 5 degrees out.

    Also, when comparing 2 lenses I sometimes can make out the letters
    easier on one lens, but the brightness/contrast on the other is much

    It's interesting to hear about the difference of opinion on the red/
    green test.

    Thanks again
    a06812, May 22, 2007
  6. a06812

    Dr. Leukoma Guest

    I think most would agree that 0.25 diopters is within the standard
    error of a refraction. Even a widget is manufactured against a set of
    tolerances, or a range of values that are acceptable.

    Dr. Leukoma, May 22, 2007
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