Distant Visual Standards -- Their origin

Discussion in 'Optometry Archives' started by Otis Brown, Aug 5, 2003.

  1. Otis Brown

    Otis Brown Guest

    Dear Kory,

    Here is a discussion of the standard distances
    used in optometry. 20/20 (feet) is the equivalent of 6/6 (meters)

    Similarly 10/10 or 3/3 indicates the same thing. It is a
    visual angle of 5 minutes-of-arc, where the distance must
    be greater that 10 feet or 3 meters.

    It is always good to double check measurements -- yourself.

    Best,

    Otis
    Engineer


    THE SCIENCE OF YOUR MEASUREMENT

    Science is the art of making measurements. Make them yourself
    and you will believe the results. The first step is to establish
    a baseline measurement for your eye.

    Cut letters out of the newspaper of the following sizes, and
    put them at 20 feet (6.1 meters). [Plan to buy the chart at a later date.]


    1.4 Inch (3.55 cm) 20/80

    3/4 Inch (1.77 cm) 20/40

    3/8 Inch (0.89 cm) 20/20


    Do not panic if your vision is worse the 20/20. It does take
    time to gradually work your way out of the situation. (i.e.,
    change your focal status from a negative value to a positive value
    by wearing a plus lens.) For the most part, when you are at 20/40 you
    can work without wearing the minus lens. For driving at night, of
    course use the minus lens, but take it off as soon as you do not
    need it. When you see 20/20 you can an end further use of the minus
    lens.

    Put several charts up around the house and at work. Be prepared
    to see variation. This is completely normal. Form an opinion of the
    line that you can read on-average.

    Now you can begin using the plus lens for all reading. Use a
    plus lens as strong as possible that just blurs your habitual
    reading distance. Periodically push the work away for complete
    blur, and then bring it in until it just clears -- and continue to
    read at that distance.

    CONCLUSION

    The standard 0.88 cm letters at 6.1 meters set by Dr. Snellen was
    recognized to be excessive. For most people, and as a legal requriement,
    1.7 cm letters (20/40) were established as the standard minimum
    requirement. Make certin you can see at this level before you
    drive a car.

    You should understand these legal requirements and make certain
    you always meet them -- by reading your own eye chart.
    Bates suggested reducing the use of a minus lens. And I agree
    with him PROVIDED you pass the 20/40 line on your eye chart.

    Some ophthalmologists health have the same idea that you do -- that the
    minus lens should be restricted, and that an effort muat be made to
    prevent the situation.

    It is relatively easy to identify the problem of the minus lens.
    It is much more difficult to transfer intellectual wisdom and
    control to you so that you will judge the choice you must make
    about your course of action. If you
    wish to become a pilot there are three possible choices
    you might make.

    1. Do nothing, and hope your vision will clear all by itself.

    2. Change your career choice.

    3. Resolve to explore the use of the plus lens by exhaustively
    reviewing the opinions and experimental data before you use
    any lens -- plus or minus.

    These choices and course of actions should be decided by
    you. Dr. Steve Leung is a optometrist who will assist you
    in making this type of determination. I know he wants to help
    you in every way possible, but I also know that ultimately, it
    is your own persistance that will achieve success in the long run.

    For the past 100 years (some) optometrists have been attempting to
    explain this choice to you for your thoughtful review. They
    are willing to assist you in your decision to use the plus lens before
    you begin using the minus lens. But you must be
    willing to take the lead in this work -- if you
    are to bcome successful with it.

    The effect of the minus lens on the natural eye is understood by
    many optometerists. They just have a hell of a job attempting
    to get you to use it properly at the 20/50 level.
     
    Otis Brown, Aug 5, 2003
    #1
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  2. Otis Brown

    Kory Postma Guest

    Law in MO, only 20/40 in one eye will be legal.

    Thanks for the info.

    Kory

     
    Kory Postma, Aug 6, 2003
    #2
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  3. Otis Brown

    Otis Brown Guest

    Dear Robert,

    What I would suggest to a person whose vision is
    at 20/40 (and entering a four year college) is that
    he better make plans to clear it to 20/30, 20/25 or
    20/20. The method I suggest is obviously the use
    of a strong plus lens for all close work.

    He should know that the eyes of West Point students
    go "down" at a rate of -1/3 dipoters per year, and
    should understand that fact as part of his
    "decision process" if he has a strong desire
    to qualify as a pilot.

    Obviously this is not an easy process.

    Best,

    Otis
    Engineer
     
    Otis Brown, Aug 6, 2003
    #3
  4. Otis Brown

    Jan Guest

    The standards notations of vision aquity in the world are 20/20 (when feets
    are used), 6/6 (when meters are used) or visus 1.0 ( frequently used in
    Europe),and they all are meaning the same.
    A shorter distance than 20 feet or 6 meters could result in an incorrect
    prescription
    For instance when a chart is positioned at 4 meters instead off 6, you have
    a 0.25 dpt error when prescribing for the far distance.
    For some people with a high vision aquity ( 20/10, 6/3 or visus 2.0) this
    might be a terrible outcome.
    --
    Jan (normally Dutch spoken)


     
    Jan, Aug 6, 2003
    #4
  5. Otis Brown

    Otis Brown Guest

    Dear Jan,

    I am not certain what you mean by "terrible outcome" as you suggest.
    Could you describe, or explain exactly what you mean by that
    statement?

    Also, the normal distance was 20 feet (considered to be "infinity").
    However, I was just explaining the meaning of 3/3, for clarification,
    and the fact that some ODs or "testers" are using that distance.

    Using the 1/f formula, then 1/3 would be 0.33 diopters -- approximatly.
    and 1/6 would be 0.166 diopters (on my slide rule, relative
    to infinity. Most do not consider this difference significant.
    Most measurements are rounded off to 1/4 diopter increments.

    The effect of prescribing (rather than checking) at 3 meters
    would be that the eye (as a box camera) would be "underprescribed"
    by 1/4 diopter (approximately). Is this a serious problem?

    Best,

    Otis
    Engineer
     
    Otis Brown, Aug 7, 2003
    #5
  6. Otis Brown

    Jan Guest

    People with a high vision aquity are capable to respond on very minor errors
    in the prescription in the same manner as "the princess on the pea" (I realy
    do not know if you are familiair with this fairy-tail)
    These people could react in a way like this " I can not see a damnn thing
    with these spectacle's, fix it"
    In that way you must see my "terrible outcome" remark.
    The refractive error you make is +0.25 dpt. (a little bit myopic) when you
    place the chart on 4 meters.

    Yes, and I want to explain why this confuse some people here by not taking
    notice off the possible errors when doing so and not knowing how to perform
    these tests as the were meant to be.
    I believe that American optometrists sometimes even use the 0.12 dpts to
    finetune.
    And as I said before, people with a high vision aquity some times have these
    "terrible outcomes" not likely the people in general.
    If you want to say it in boxing terms (grinsss...) the box is a little bit
    myopic and make a blurred picture when the horizon is shooted.
    In this case whe agree both that there is no "depth off field", okay?

    And no, not a serious problem too me, I have such a high vision aquity, but
    it might be for others.
    My vision aquity OD=20/10 OS=20/10 when corrected for hyperopia/myopia in
    one eye and the hyperopia/hyperopia in the other.
    Mother nature (and my parents) provided me with this high vision aquity.
    BTW, I am also presbyopic due to my own stupidity. (but with a smile on me
    face and relaxed)

    Jan (normally Dutch spoken)
     
    Jan, Aug 7, 2003
    #6
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