Difference between 5" letters at 20' and 10" letters at 40'?

Discussion in 'Optometry Archives' started by LurfysMa, Aug 17, 2005.

  1. LurfysMa

    LurfysMa Guest

    Is there a difference to the eye between 5" letters at 20', 10"
    letters at 40', 20" letters at 80', etc.?

    LurfysMa, Aug 17, 2005
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  2. Mike , do you mean that it is same when a person uses 20/10 line at 10
    feet for 20/20 and 20/20 line at 20 feet? What could be the peripheral
    and atmospheric cues? Lighting?

    Yasar, Mehmet PFC A Co 602d ASB, Aug 17, 2005
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  3. LurfysMa

    otisbrown Guest

    Dear LurfysMa,

    Subject: Eye-chart letter size, and resolution
    of the eye.

    Re: How this test was set up by Dr. Snellen

    Re: Is there a difference to the eye between 5" letters at 20', 10"
    letters at 40', 20" letters at 80', etc.?

    The intention of this "Standard" was to
    have eye's checked at 20 feet (or 6 meters).

    It was judged that 6 meters was far enough to be considered "optical
    infinity" for the eye.

    The human eye ( under good circumstances)
    can "resolve" to 1 minute-of-angle.

    Snellen made the letters 5 minute-of-angle in size, which makes the
    letters 0.9 cm at 6 meters.

    Keeping the angle constant, then this (20/20) letter would be 1.8 cm at
    12 meters.

    Thus you are measureing the resolving power of the human eye by this

    For eas of use, some charts can be designed for 3 meters. (Small
    office.) Then, you could say 10/10 -- to be technically correct.



    otisbrown, Aug 17, 2005
  4. LurfysMa

    LurfysMa Guest

    That was my thinking, too, but it doesn't seem to work that way.

    I have been trying on contacts the past few weeks. I have been using
    reading glasses for 20 years, but never had a problem with distance. I
    swear I used to be able to read license plates a block away (I wish I
    had taken good measurements).

    I went in for an annual eye exam and the doctor told me I was
    borderline for passing a DMV eye test. I had noticed that distance
    vision was not as good, but I guess I hadn't realized how much it had

    Anyway, I went to see about contacts. The optician used the standard
    eye chart to determine which lenses were good for me. It seems to me
    that lenses that make the bottom line on the eye chart clear do not
    make it easer to read street signs and license plates at much longer

    I've read here about focal length. Is that a factor?

    We tried monovision and that didn't work at all for me. I might not
    have given it a long enough trial, but I couldn't see well enough to
    drive safely. Close up vision was great.

    I now have distance-only contacts. They seem to help the most at
    intermediate range, but not much at long distance.

    The optician said that far-sighted people are the hardest to fit.
    They are used to being able to see gnats on mountain tops and they
    aren't happy with anything less.

    Any suggestions? I need to go in next week for another fitting.

    LurfysMa, Aug 17, 2005
  5. LurfysMa

    otisbrown Guest

    Dear Robert,

    Thanks for your input.

    The word "emmetropia" has several meaings, including refractive
    states running from -1/2 diotper to +3/4 diopter -- unless you
    specify that "emmetropia" is a refractive state of EXACTLY

    The remark of the 10 foot (3 meter) is just the simple
    fact that some doctors use is for simple convenience.

    The "offical" chart remains at 20 feet, and refractive
    status to be measured with a trial-lens kit (or


    otisbrown, Aug 17, 2005
  6. LurfysMa

    Quick Guest

    !! I've never thought of that. I always assumed the moon
    looked larger low in the sky due a magnification effect of
    the atmosphere. Low in the horizon you are seeing it though
    more atmosphere than overhead. I'll have to measure the
    image next time.

    Quick, Aug 17, 2005
  7. LurfysMa

    otisbrown Guest

    Dear LurfysaMa,

    Just for the fun of it you might enjoy checking your DMV-Snellen at:


    Just type 1 inch for line-length, and 10 feet for distance
    and see what you can read.

    The DMV generallly requires 20/40 or better in most states.

    If the "contacts" bother you, and you pass the DMV, you could
    go back to using (plus) reading lenses.


    otisbrown, Aug 17, 2005
  8. LurfysMa

    Don W Guest

    Just for the fun of it you might enjoy checking your DMV-Snellen at:
    Otis, For some reason (OK, the design) they have a horizontal split in the
    screen that make the screen difficult to scroll.

    Don W
    Don W, Aug 18, 2005
  9. LurfysMa

    Quick Guest

    Ummm, getting back to earth... If I have a 1 mm segment
    in my contact lens, what is the size of the image that would
    be covered at 30"?

    Quick, Aug 18, 2005
  10. LurfysMa

    otisbrown Guest

    Dear Don,

    Subject: Snellen Chart for Your Monitor.

    The "Snellen" does not "scroll". Just click on "Display" to get
    a new set of random letters.

    You can change the size of the letters by clicking
    on "Larger" and "Smaller".



    otisbrown, Aug 18, 2005
  11. Using simple geometry and some fairly crude assumptions/estimations I
    get about 30 mm or 12 inches. That of course would be best case
    scenario, and assumes perfect (infinitely thin) ledges/lines. Your
    mileage will vary...

    w.stacy, o.d.
    William Stacy, Aug 18, 2005
  12. LurfysMa

    Quick Guest

    Perfect! That should just about cover the viewable area of my
    20" CRT... Although I guess you don't really look at that much
    at one time? Seems that if I look at a spot or word of text anything
    outside of about a 4" diameter circle is sort of "peripheral"
    to what you are looking at and you can't really tell if it's in focus
    or not. You have to shift your gaze past that.

    Surely this has been quantified/characterized?

    (should I have started a new thread? or is this still
    on topic for this one?)

    about 30 mm or 12 inches. That of course would be best case scenario, and
    assumes perfect (infinitely thin) ledges/lines. Your mileage will vary...

    w.stacy, o.d.

    Quick wrote:
    Ummm, getting back to earth... If I have a 1 mm segment
    in my contact lens, what is the size of the image that would
    be covered at 30"?


    Robert Martellaro wrote:

    On Wed, 17 Aug 2005 13:52:19 GMT, "Mike Tyner"

    "Peripheral" cues are all those other surrounding
    objects, relative motion, relative size - for instance,
    why the moon looks bigger when it's low in the sky.

    Although this explanation has merit, the primary effect
    is a physiological one.
    As one tilts his head back to observe the moon at the
    zenith, the lens of the
    eye is flattened by gravity and its magnifying power is
    decreased. (Thanks to
    the folks at sci.astro)

    Robert Martellaro
    Roberts Optical

    "An expert is a person who has made all the mistakes that
    can be made in a very narrow field." - Niels Bohr
    Quick, Aug 18, 2005
  13. You could do that at any time (like I just did). I don't think trifocal
    contact lenses have been studied much. Good luck with them. Sounds like
    you're on the right track. Let us know how it works out, since you are
    probably a pioneer in this area.

    w.stacy, o.d.
    William Stacy, Aug 19, 2005
  14. LurfysMa

    Quick Guest

    doh... all those business terms come to mind. "opportunity",
    "interesting problem". I was starting to get clues though.
    "maybe only at handful have fitted those in the US" and
    when Fused Kontacts told me they didn't have a fitting kit
    for the trifocals but they did have very good consultants
    that would work directly with my OD to design the lenses.

    I'm looking forward to it. Seems there is a large hole with
    bifocals. One can go with multifocal simultaneous vision
    solutions which I've come to gather all have "compromises"
    that are significant. Sacrifice off of both ends or significantly
    off of one end or the other or resort to monovision. The flare,
    "soft" vision and other anomolies. I was a bit surprised this
    hasn't all been sorted out some time ago.

    I'm a slightly hyperopic (~+1.00) presbyope (~+2.00) who
    sits in front of a computer all day. I've always been very
    picky about my vision. I guess I could be in the worst case
    section of the fitting text book. The whole idea is to do
    without spectacles. I mean, that's what contacts are for right?
    I can do without correction just fine for distance although
    I do miss the sharpness of being able to see the tree
    line on a mountain ridge a few miles away. I want to get
    rid of the readers. If I have to insert and remove contacts
    depending on what I'm doing then we're back to readers.
    So my goal is perfect distance, perfect reading, and perfect
    computer (~30"). I'm willing to compromise at about 5 feet
    which is the distance between my nose and the golf ball
    on the tee since I can't golf like I used to anyway (and I've
    accepted that :)). Besides, once you're lined up you should
    be able to swing with your eyes closed. I think I can deal
    with adapting to moving my head more than my eyes most
    of the time.

    So it seems that all the stuff is aimed at broad range with
    "compromise" everywhere or 2 focal points, distance
    and some point closer. I would have thought 3 focal points
    to be the minimum requirement and more would have
    been done with trifocals already?

    Quick, Aug 19, 2005
  15. As you probably know, spectacle trifocals are quite common and
    appreciated by many folks who have zero or close to zero accommodative
    amplitude, and for those 3 focal lengths is necessary/useful.

    The problem with trifocals that when either (or both) seg line infringes
    on the pupillary area, there is a certain amount of light scatter and/or
    double refraction at and around the line(s).

    Our theoretical clear zone of 12 inches with the 1 mm intermediate seg
    on the TS Tri would be well overlapped by interference from both
    distance and near zones, since the pupil size is probably on the order
    of 3 mm. Hopefully the "main" clear image through the intermediate seg
    would overshadow the other "focuses" to the extent that you won't have
    appreciable degradation of your vision. YMWV

    At least with the TS bi you can get the pupil fully covered by the
    distance area, with no interference from the seg when viewing distance
    objects, and vice-versa with near. What's probably kept me from trying
    the TS tri is that I'm pretty much guaranteed *some* interference in
    that mid seg.

    But however much interference there is from the lines themselves and
    from the other focal areas, I'm willing to bet that it will still give
    better vision than you'd get with ANY of the aspheric (smeared optics)
    alternatives, and maybe, just maybe it won't even be noticeable. That's
    why I'm interested in your input, as I always like to hedge my bets.
    Again, you'll be the first I ever encountered who has tried them, and
    I've been around a while (have fit a dozen or so TS Bis over the years).

    w.stacy, o.d.
    William Stacy, Aug 19, 2005
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