Diopters to 20/something conversion. The math and science behind this!

Discussion in 'Optometry Archives' started by acemanvx, Jan 10, 2006.

  1. acemanvx

    acemanvx Guest

    Hello everyone, post your dioptric pescription(how bad are your eyes),
    your UCVA(uncorrected) and BSCVA(corrected by glasses) Also if you know
    anyone's vision, post that too. I have done much research on the
    correlation of diopters and 20/xxx and have compiled several charts,
    formulae, tables and comments on the results obtained thus far. To test
    for this, do it in room light or dim light. Make sure the eyechart is
    well illuminated and your not squinting at all or recalling from memory
    or guessing. Blur preception is of course allowed.

    I and most optometrists have found that -.25 diopters doesnt cost you a
    single line but makes the existing line blurry but still readable. Some
    say it costs half a line. Such as if you can see half of 20/15 with a
    -.25 lens, youll be a full 20/20 but wont be able to see better than
    that uncorrected. Another example is if your seeing all of 20/20, you
    may miss half of the 20/20 with -.25 diopters. Others have said it
    didnt affect their ability to read a line, just made it harder to do
    so.

    Minus half diopter(-.5) is generally accepted as resulting in one full
    line loss. This means 20/15 becomes 20/20 and 20/20 becomes 20/25. If
    you can see half of 20/15 youll be seeing half of 20/20.

    Minus one diopter(-1) generally gives you half visual accuracy. My
    friends who see 20/20 with glasses see 20/40 without their -1 glasses.
    Ditto for 20/15 with -1, 20/30 uncorrected. I was 20/50 corrected to
    20/25 with a -1 lens years ago.

    Higher dioptric values become harder to calculate and predict,
    especially when you get to -6 and up. Someone could be -4.5 another
    -5.25 and both see 20/400 UCVA. The -5.25 probably will see a much
    blurrier 20/400 than the -4.5 but generally, 20/400 represents a
    moderate of myopia around -5 diopters. One website said the range was
    -4 to -6 for 20/400 and your best corrected vision played a big factor.
    Someone whos 20/15 corrected needs more diopters to see the same blur
    as another with 20/30 corrected. For me, my left eye at -5 or so
    couldnt see 20/400 while my right eye at -4.5 or so just barely, barely
    saw 20/400. I know two people who passed V3 requirement which states
    you have to be 20/400 or better uncorrected and both were barely 20/400
    with -5.5 pescriptions with 20/15 corrected. Those two guys said they
    dont know anyone else -5.5 or more who passed V3 which requires 20/400.
    They probably didnt have the great 20/15 corrected vision with glasses
    so for them, -5 was the limit(20/20 corrected) I am not correctable to
    20/20 so my limit is even less.

    Few eyecharts go beyond 20/400 so info is scarce past this. I have a
    solid grasp on dioptric values below -6(mild to moderate myopia), but
    for -6 and up which is high myopia I am less certain how this converts
    to diopters. I do know high myopes are worse than 20/400 but how bad
    exactly? Also theres much fewer high myopes than low and moderate
    myopes so much of my info is on low(er) myopes and their diopters to
    snellen accuracy.

    One could stand closer but then accomodation needs to be taken into
    account. I can see the 20/200 E from 10 feet less blurry than the
    20/400 E from 20 feet. At 5 feet I see the 20/70 line but im certainly
    worse than 20/280. I experienced .67 diopter accomodation from the 5
    feet mark.

    If any of you have charts and formulae, on what expotentional scale
    does this equal to? I know that going from -1 to -2 is only twice as
    bad(20/40 and 20/80), but going from -3 to -6 is definately more than
    twice as bad(20/150 and 20/500?), blurs much more than twice. Does this
    gap become even larger still at -6 to -12(20/500? and 20/????) where
    the number of times more blurry increases expotentionally?

    Me and many others are curious about this and this topic has been
    posted before many years ago. Its also useful for some occupations such
    as pilot, soldier, law enforcement, etc where they have a requirement
    for both corrected(BCVA) and uncorrected(UCVA) vision and people ask
    all the time if their UCVA is good enough to make the cut.
     
    acemanvx, Jan 10, 2006
    #1
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  2. acemanvx

    acemanvx Guest

    Let me attempt to re-arrange the paragraphs to make reading easier. It
    didnt show up right


    Hello everyone, post your dioptric pescription(how bad are your eyes),
    your UCVA(uncorrected) and BSCVA(corrected by glasses) Also if you know
    anyone's vision, post that too. I have done much research on the
    correlation of diopters and 20/xxx and have compiled several charts,
    formulae, tables and comments on the results obtained thus far. To test
    for this, do it in room light or dim light. Make sure the eyechart is
    well illuminated and your not squinting at all or recalling from memory
    or guessing. Blur preception is of course allowed.

    I and most optometrists have found that -.25 diopters doesnt cost you a
    single line but makes the existing line blurry but still readable. Some
    say it costs half a line. Such as if you can see half of 20/15 with a
    -.25 lens, youll be a full 20/20 but wont be able to see better than
    that uncorrected. Another example is if your seeing all of 20/20, you
    may miss half of the 20/20 with -.25 diopters. Others have said it
    didnt affect their ability to read a line, just made it harder to do
    so.

    Minus half diopter(-.5) is generally accepted as resulting in one full
    line loss. This means 20/15 becomes 20/20 and 20/20 becomes 20/25. If
    you can see half of 20/15 youll be seeing half of 20/20.

    Minus one diopter(-1) generally gives you half visual accuracy. My
    friends who see 20/20 with glasses see 20/40 without their -1 glasses.
    Ditto for 20/15 with -1, 20/30 uncorrected. I was 20/50 corrected to
    20/25 with a -1 lens years ago.

    Higher dioptric values become harder to calculate and predict,
    especially when you get to -6 and up. Someone could be -4.5 another
    -5.25 and both see 20/400 UCVA. The -5.25 probably will see a much
    blurrier 20/400 than the -4.5 but generally, 20/400 represents a
    moderate of myopia around -5 diopters. One website said the range was
    -4 to -6 for 20/400 and your best corrected vision played a big factor.
    Someone whos 20/15 corrected needs more diopters to see the same blur
    as another with 20/30 corrected. For me, my left eye at -5 or so
    couldnt see 20/400 while my right eye at -4.5 or so just barely, barely
    saw 20/400. I know two people who passed V3 requirement which states
    you have to be 20/400 or better uncorrected and both were barely 20/400
    with -5.5 pescriptions with 20/15 corrected. Those two guys said they
    dont know anyone else -5.5 or more who passed V3 which requires 20/400.
    They probably didnt have the great 20/15 corrected vision with glasses
    so for them, -5 was the limit(20/20 corrected) I am not correctable to
    20/20 so my limit is even less.

    Few eyecharts go beyond 20/400 so info is scarce past this. I have a
    solid grasp on dioptric values below -6(mild to moderate myopia), but
    for -6 and up which is high myopia I am less certain how this converts
    to diopters. I do know high myopes are worse than 20/400 but how bad
    exactly? Also theres much fewer high myopes than low and moderate
    myopes so much of my info is on low(er) myopes and their diopters to
    snellen accuracy.

    One could stand closer but then accomodation needs to be taken into
    account. I can see the 20/200 E from 10 feet less blurry than the
    20/400 E from 20 feet. At 5 feet I see the 20/70 line but im certainly
    worse than 20/280. I experienced .67 diopter accomodation from the 5
    feet mark.

    If any of you have charts and formulae, on what expotentional scale
    does this equal to? I know that going from -1 to -2 is only twice as
    bad(20/40 and 20/80), but going from -3 to -6 is definately more than
    twice as bad(20/150 and 20/500?), blurs much more than twice. Does this
    gap become even larger still at -6 to -12(20/500? and 20/????) where
    the number of times more blurry increases expotentionally?

    Me and many others are curious about this and this topic has been
    posted before many years ago. Its also useful for some occupations such
    as pilot, soldier, law enforcement, etc where they have a requirement
    for both corrected(BCVA) and uncorrected(UCVA) vision and people ask
    all the time if their UCVA is good enough to make the cut.
     
    acemanvx, Jan 10, 2006
    #2
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  3. acemanvx

    Dom Guest

    Aceman this is an amazing question, not least because of its length. I
    really am curious about you... how does one develop such an interest
    (obsession) with vision and vision correction? Even optometry students
    don't have your keen level of amateur enthusiasm. Do you have other
    interests? What is your age? Do you work or study (I'm guessing not, as
    you clearly spend a great deal of time on the internet researching these
    topics - maybe too much time).

    This is a genuine question to find out how a person can be so obsessed
    with something (the process of prescribing glasses and other vision
    correction) that most people just don't find that interesting!

    You are obviously a thinker but remember the old expression, "a little
    knowledge is a dangerous thing".

    Dom
     
    Dom, Jan 10, 2006
    #3
  4. acemanvx

    Don W Guest

    Just wondering, what limits the amount of interest one may have in certain
    areas?
     
    Don W, Jan 10, 2006
    #4
  5. An absence of OCD?
     
    Scott Seidman, Jan 10, 2006
    #5
  6. acemanvx

    acemanvx Guest

    As to what Otis said, since most are myopes, the minus lens is tried
    first, if this makes things blurrier the person could be a hyperope and
    a high one at that if hes young. Test with plus lenses then. Then
    theres the cylindar component which is tricky and many optometrists
    tend to give spherical equivalent with more minus or less plus. The
    pinhole test is a quick diagnosis to check for refractive error or
    occular pathalogy.

    someone said:

    "the real answer depends on the optics of corneal curvature vs axial
    length
    - an eye with steep cornea and average length can be equally
    nearsighted
    with an eye with a flat cornea that is very long - but they may not
    have
    the same uncorrected acuity."


    I dont see how this makes a difference be it the cornea, lens, axial
    length or even giving someone a plus lens(other than magnificarion) all
    refractive error is expressed in units and blur equally. I never heard
    of different types of components resulting in refractive error to
    affect the final outcome.


    "d=log(20/xxx)/.27"

    This formula only works somewhat for a narrow range. I havent composed
    my own formula, but just a table with the values.


    You also have a database with 150 results and said It looks to me like
    the only people this works well for have 20/20 uncorrected acuity.

    I really, really question all those -.25 guys with 20/40, 20/50, 20/70
    or even 20/100 accuracy who "magically" see 20/20 or 20/25 with minus
    one quarter diopter! Sorry but this is not possible. I have seen the
    differences of quarter diopters with trial lens and when I got my eye
    exam thru phororapter. The differences is very small, sometimes I have
    to ask the optometrist to try two, three, four times flipping between
    quarter diopters to determine which is better. My mom has tried -7.5
    and -8 contacts and said there wasnt a difference between the two! She
    of course wears -7.5 now since more minus doesnt help.

    I also question some of the seemely poor vision with very low dioptric
    values(see above) and seemly good accuracy with high diopters.
    Squinting perhaps? Overminus? -3 diopters is going to be alot worse
    than 20/80 if BCVA is 20/20.


    Anyway I already saw those threads from years ago. This thread will be
    like a countinuation and add new results. You optometrists get to know
    results everyday when doing eye exams. Im sure youve noticed a pattern
    by now excluding the odd ones which dont make sense like -.25 and 20/70
    or -4 and 20/70 for example.
     
    acemanvx, Jan 10, 2006
    #6
  7. acemanvx

    Dom Guest

    To answer your question: I suppose things like:
    The amount of time available to spend reading, learning & talking about it.
    Access to resources to further one's interest.
    Competing interests, hobbies, and other activities.
    Time spent on daily necessities such as work, study, eating, sleeping etc.
    I'm sure you can think of others.

    .... but my original question was trying to find out how or why Aceman
    developed such a keen interest in this topic... trying to find out a
    little about the person behind all the posts.

    Dom
     
    Dom, Jan 10, 2006
    #7
  8. We have shown you that most people ARE NOT myopes, or did you forget that?

    w.stacy, o.d.
     
    William Stacy, Jan 10, 2006
    #8
  9. acemanvx

    acemanvx Guest

    Reguardless, the minus lens is tried first by default then the plus. I
    do see more old people being farsighted but much more young people are
    nearsighted. low hyperopia is asymptomic for young people so if a young
    person had 20/60 vision, its much liklier that hes myopic.
     
    acemanvx, Jan 10, 2006
    #9
  10. It's obvious that in your wide ranging reading about myopia etc you
    never ran across a book on refraction techniques. NONE OF THEM, NOT ONE
    recommend that. Based on physiological optics, it is EXACTLY THE WRONG
    THING TO DO. All experts start with at least +1.50 or +2.00 and go from
    there (usually with a retinoscope).

    You're recommending the stupidest thing I've ever heard in refraction
    technique, but then that's why I have the license and you don't, thankfully.

    w.stacy, o.d.
     
    William Stacy, Jan 10, 2006
    #10
  11. acemanvx

    Guest Guest

    Nonsence, layman talk.
    If this was true , you did not have vision problems ace.
    Overminusing works the same way as low hyperopia as you already find out.
    Think about headaches, total fatigue, lack of concentration power etc.
    Speaking of concentration ace, hands above the sheets!

    And ace, whenever you respond it is internet etiquette to show to whom you
    respond (see the top of this message)

    --
    Free to Marcus Porcius Cato: ''Ceterum censeo Carthaginem esse delendam"

    In conclusion, I think that the "Otis therapy" should be destroyed

    Jan (normally Dutch spoken)
     
    Guest, Jan 10, 2006
    #11
  12. acemanvx

    otisbrown Guest

    Dear William,

    Subject: Refractive state of the natural eye -- in the open.

    Re: Refractive states dependent on the average visual enviroment --
    measured
    directly and objectively -- by Francis Young.

    It is true that the natural eye -- in the wild will have
    positive refractive states running from zero to plus-2 diotpers.

    Measured with a Snellen and Trial-lens frame. Measurement
    as previously described.

    For the primates, these refractive states form a gaussian
    distribution with an average value of about +0.75 diopters.

    Best,

    Otis
     
    otisbrown, Jan 11, 2006
    #12
  13. acemanvx

    Dom Guest

    No we don't. We tend to find the cylinder. It's not tricky at all if
    you're an optometrist.

    Who are the "-.25 guys with 20/40 [to] 20/100"? Where have you heard or
    read about them? I haven't come across these people.


    Remember, Ace, a little knowledge is a dangerous thing. You have a
    little knowledge. Be careful that you don't present yourself as having a
    comprehensive knowledge.

    Dom
     
    Dom, Jan 11, 2006
    #13
  14. acemanvx

    acemanvx Guest

    My pre-op Rx and post-op results are:


    OS (left) OD (right)

    Pre-op -9.00 sph +1.00 cyl -8.75 sph +1.75
    cyl


    2 weeks post-op -2.25 sph 0 cyl -1.75 sph 0 cyl


    6 weeks post-op -3.25 sph 0 cyl -2.75 sph 0 cyl


    The surgery only took about 10 minutes and was relatively painless.
    The
    worst part was when they examine the cornea afterword with a VERY
    BRIGHT
    light. You can't blink and the light is painfully bright, but it
    doesn't
    last too long. The days and hours of anxious waiting before the
    surgery
    is far worse than the actual procedure.


    I had absolutely no post-op discomfort and haven't since, except for an

    occasional dry burning sensation that goes away with eye drops. As you

    can see from my results, I didn't get 20/20 vision on the first try. I

    saw 20/70 the next day and have slowly regressed to about 20/100 or so.

    Note to all you high myopes: 20/70 is damn good compared to whatever we

    are at -9 or worse! Before, I couldn't read the big E on the eye chart.

    Now, even with -3 d of nearsightness I can function without glasses,
    which is something I could not do before, so it is a big improvement.

    My comments: Hes giving the correlation between diopters and 20/xxx
    notice
    hes seeing 20/70, presumbly with the better -1.75 eye. Also notice he
    regressed
    to 20/100 then a little worse, perhaps 20/150 at -2.75
    also notice at -9 theres no way he will see any snellen eyechart
    letters
    not even the 20/800 E! -3 is a very, very big improvement when you used
    to be
    a -9! Hes probably going to leave things well enough alone and not go
    for
    a lasik enhancement due to his presbyopia and keeping his near vision
    intact.
    Its not as much improvement for me where im a -4.5 and using a -1.5
    trial lens gets me down to -3 but enough improvement to be noticable
    but not quite enough to go "wow!" Give me a -3 trial lens and were
    talking
    a significent improvement and I wish I were only -1.5 diopters! This
    would be good enough to almost never need distance glasses, yet not
    need
    reading glasses except for fine print.
     
    acemanvx, Jan 11, 2006
    #14
  15. acemanvx

    acemanvx Guest

    "No we don't. We tend to find the cylinder. It's not tricky at all if
    you're an optometrist."


    Youd be supprised how many times I get just the sphere without
    cylindar. Last eye exam nearly a year ago he gave me -5.5x-.5(left) and
    -5(right) but this isnt right and is even more off now that my eyes
    have gotten slightly less nearsighted(reduction of the pseudomyopia
    component) My right eye has a sphere closer to -4 with -.75 cylinder.


    "Who are the "-.25 guys with 20/40 [to] 20/100"? Where have you heard
    or
    read about them? I haven't come across these people."

    It was in William's spreadsheet in previous posts from years ago when
    he tested diopters vs. 20/xxx

    "Remember, Ace, a little knowledge is a dangerous thing. You have a
    little knowledge. Be careful that you don't present yourself as having
    a
    comprehensive knowledge."

    Just a young man learning about vision and optics =) :) :D


    so anyway do you have any experience on diopters and 20/xxx?
     
    acemanvx, Jan 11, 2006
    #15
  16. acemanvx

    Dom Guest

    If your cylinder result varies from test to test, it's more likely due
    to variations in your tear film, your accommodation, or your responses,
    rather than the optometrist not being capable or willing to find the
    cylinder. In my experience, those patients who are the most concerned
    about giving all correct answers, analysing each choice carefully,
    viewing the alternatives multiple times, and not making any mistakes,
    are those the most likely to end up with a *less* accurate refraction as
    a result. Perhaps this could have been your experience.
    I don't know what spreadsheet you're referring to... does this
    spreadsheet really claim to record eyes with UCVA of 20/100 and
    sphero-cylindrical refraction of -0.25sph, giving BCVA of 20/20? Or is
    there more to it than that?
    Yes it's great to ask questions and learn, and all the optometrists on
    this newsgroup will happily answer reasonable questions BUT be careful
    you don't present yourself as an expert or bore people with long-winded
    rambling posts, or you risk being thought of as a "know it all" who
    really knows very little.
    Yes plenty. But I haven't recorded it or worked out any formulae. That's
    because the value of any such formula would be extremely limited to say
    the least. Here are a few clues as to why: "pupil size; hyperopia;
    astigmatism; accommodation; lens opacities; tear film quality;
    motivation; pathology".

    Dom
     
    Dom, Jan 11, 2006
    #16
  17. I think you misread that tally, or maybe somebody clipped off the
    cylinders. I have never had many -.25 sph patients with 20/40 to 20/100
    (there are always a few with macular degeneration, etc.) No healthy eye
    with -.25 sph is anywhere near 20/100

    w.stacy, o.d.
     
    William Stacy, Jan 11, 2006
    #17
  18. Years ago on this n.g. the subject line above question had come up and
    had been argued for so long that I decided to query my database (I was
    kindof a pioneer in recording refractions and acuities directly into my
    PC as I did the exam), and tallied refractions against unaided acuities.
    I haven't seen that particular tally lately, but I know it got
    manipulated a few times as some of the other contributers fit the
    equation to the curve. I called it the "Dead Horse Equation" because
    the subject was posted so many times that I thought we were kicking a
    dead horse around, trying to get it to work. Anyway, it was an
    interesting exercise, and I must say most contributors at that time
    agreed that it was a valiant attempt to make the corellation, there are
    so many variables in real life, it was not a very useful thing, and I
    don't think it ever made it into the curricula of any optometry
    schools... Anyway, if there were any 20/100 -.25 people, they certainly
    had eye disease or large astigmatism.

    w.stacy, o.d.
     
    William Stacy, Jan 11, 2006
    #18
  19. acemanvx

    acemanvx Guest

    Your spreadsheet showed many people being corrected to 20/20 or 20/25
    and I dont think anyone worse than 20/40 as I recalled. Curiously, none
    corrected better than 20/20 despite you now saying otherwise. In that
    spreadsheet, it showed some -.25 guys with 20/40 to 20/100 UCVA and
    20/20 or 20/25 BCVA. This sounds like an error or something. Perhaps I
    am confused when reading your spreadsheet. In reality, I would think
    -.25 diopters shouldnt cost even one line, lots of people are still
    20/20 with -.25 and some at -.5 for the matter. How do you find
    someone's UCVA if they cant see the 20/400 E on the projection
    eyechart? in your experience(and the experience of any other
    optometrists here), what diopter values corresponded with what visual
    accuracies?

    such as:

    -1
    -1.5
    -2
    -3
    -4
    -5
    worse than -5
     
    acemanvx, Jan 13, 2006
    #19
  20. I think my habit at the time was only to test to 20/20. That was over
    10 years ago. The projectors I now use are very convenient to test
    20/15 or 20/10.

    By the way, I looked at some of those on line charts you can print
    yourself, and some if not most are very crude and have letters that
    don't conform to the snellen standard construction. If you use those,
    esp. in improper lighting, you will get incorrect acuities.

    In reality, I would think
    It may or may not, and you are not in a position to determine any cause
    and effect relationships on individuals. You don't have the knowledge
    or the equipment or the skills.


    How do you find
    It's very easy, but I'm not going to tell you. Find out yourself.
     
    William Stacy, Jan 13, 2006
    #20
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