Glasses for Computer use.

Discussion in 'Glasses' started by grahamew, Mar 9, 2007.

  1. grahamew

    grahamew Guest

    I'm blowing in to ask if anyone can advise me. A year ago I got my
    first pair of reading glasses. I haven't been using them for computer
    work but lately I've had trouble focusing on the screen and I've been
    getting headaches. My reading glasses are too strong though and I
    don't like using them on the computer, I've tried them a few times.

    The screen is 24" (.61 m) from my eyes. My prescription is .125 both
    eyes, nothing else. The optician said it was age typical presbyobia.
    I don't want to pay another visit to the optician's just yet - next
    time I go, and if I need new glasses I will get them for computer use
    and get OTC readers for reading as I spend more hours doing the
    former. But until then should I get .100 or .75? OTC readers for
    computer work? And how do you work it out? I understand .100 means
    lengthening the focal point one metre.

    grahamew, Mar 9, 2007
    1. Advertisements

  2. grahamew

    Mark A Guest

    I assume your reading Rx is +1.25 diopters since +.125 is too small of a
    power bother with glasses, and most lenses only come in increments of .25.

    Optometrists, like all doctors, take pride in their sloppy and illegible
    handwriting (without the proper decimals), which is why you are confused
    about the power.

    I would just try on some OTC readers at the store, focusing on the same
    distance as your monitor (take a tape measure with you), and pick out the
    right power that way. Assuming that your reading Rx is +1.25, then your
    guess of +.75 or +1.00 is probably correct for computer distance, but you
    can easily verify that at the store.
    Mark A, Mar 9, 2007
    1. Advertisements

  3. I think you're slipping decimals. 1.00 Diopter means a focal length of
    1 meter. Your Rx may be 1.25, but it could be .125, there is a huge
    difference. The formula is 1/d=f where d=lens power in diopters and
    f=focal length in meters.
    William Stacy, Mar 9, 2007
  4. grahamew

    Dr. Leukoma Guest

    I would say that the goal of reading glasses is to supplement his
    flagging accommodation in order to bring the plane of focus closer to
    the retina, thereby reducing the area of blur or defocus.

    Dr. Leukoma, Mar 10, 2007
  5. grahamew

    Salmon Egg Guest

    Does not what I suggested above do just what you say needs to be done?

    -- Fermez le Bush--about two years to go.
    Salmon Egg, Mar 11, 2007
  6. grahamew

    Dr. Leukoma Guest

    Yes, Bill, but the devil is in the details. The conditions at
    infinity are simply not the same for everybody, i.e. nearsighted folks
    vs. farsighted folks.

    Sure, I 'm being picky, but that's my job.

    Dr. Leukoma, Mar 11, 2007
  7. grahamew

    Salmon Egg Guest

    Presumably, the 1.25D he already uses takes care of spherical correction he
    needs to compensate for his position along the myopia-hyperoia axis. The
    remainder allows him to see his close computer display clearly. This is
    simple physics--not complicated biology.

    -- Fermez le Bush--about two years to go.
    Salmon Egg, Mar 11, 2007
  8. grahamew

    Dr. Leukoma Guest

    Simple physics assumes a linear 1:1 relationship between accommodative
    stimulus and accommodative response. It also assumes that what the
    patient perceives as clear is the same as having the circle of least
    confusion exactly at the plane of the retina. If you are trying to
    paint a mental picture, it's nice to be able to have an understanding
    of the eye's physiology, which is physics and biology.

    Dr. Leukoma, Mar 11, 2007
  9. grahamew

    Salmon Egg Guest

    Educate me if I am wrong. Do you not fit spectacles so that there is a
    distance for which accommodation effort is not required? For example, does
    an emmetropic eye use accommodation to view objects at infinity? Why not
    design (fit) spectacles so that no accommodation is required at the working
    distance to the display?

    -- Fermez le Bush--about two years to go.
    Salmon Egg, Mar 12, 2007
  10. grahamew

    Dr. Leukoma Guest

    The average amount of tonic accommodation for an average eye at
    infinity, i.e. no stimulus to accommodation, is approximately 0.50
    diopters. At 40 cm., or a stimulus of 2.50 diopters, the amount of
    accommodation in a non-presbyope would be approximately 1.75
    diopters. In an early presbyope, it may be 1.00 diopter of
    accommodation. Therefore, in a patient with 1.00 - 1.25 diopters of
    accommodation at 40 cm., should I give a 2.50 diopter add, or a 1.25 -
    1.50 diopter add?

    Dr. Leukoma, Mar 12, 2007
  11. grahamew

    grahamew Guest

    Thanks for your replies, everyone. Yes, that was a typo, I meant to
    type. +1.25. Bill, are you saying I should get 3.0 glasses for the
    computer? That makes no sense to me as they would be much stronger
    than my reading glasses.

    William I took your formula, and with a bit of algebra I deduced the

    - assuming my 1.25 readers are adjusted for a reading distance of .
    355m (14 inches - I guess that's about right)

    - my screen is .61m, IOW .255m longer focal length than my readers (.

    - 1/d = f is 1/1.25=.8

    - this can be rearranged to 1/f=d (if you multply both sides by d and
    divide both sides by f)

    - I want my computer glasses to f+.255m

    - .8 + .255 = 1.055 (focal length adjusted for screen)

    - 1/1.055 = .947 = d

    So I should get 1.00 for the screen then? That I can get at Walgreens
    et al. If I get +0.75 I would have to buy online cause I haven't seen
    any that power on the shelf.
    grahamew, Mar 12, 2007
  12. grahamew

    Salmon Egg Guest

    You are obfuscating by using jargon. Do you do that to your patients? The
    meaning of the term "tonic" is not clear to me. My best interpretation of
    what you say is that an average [normal] eye is slightly hyperopic and
    "tonic accommodation" adds +0.5D of additional power to see objects at
    infinity clearly.

    In any event, if it is good practice to include the effects of tonic
    accommodation at infinity, what I said still holds. The original poster told
    us, perhaps incorrectly as suggested by previous responders, what his
    prescription was. He also told us what the distance to the computer screen
    is. Adding the reciprocal of that distance to his prescription moves that
    screen to infinity and he still uses the tonic accommodation to function

    -- Fermez le Bush--about two years to go.
    Salmon Egg, Mar 12, 2007
  13. grahamew

    Dr. Leukoma Guest

    Gee, Bill, I'm just trying to be accurate without obfuscating you. I
    would like to say that you over-simplify a complex biological system
    because you are an engineer. Words such as infinity do have
    meanings. So much for trying to expand your vocabulary.

    Dr. Leukoma, Mar 12, 2007
  14. grahamew

    Salmon Egg Guest

    Never take my advice in lieu of a vision professional. In addition, from
    your post quoted above, I get a different picture of your situation. Your
    first post gave me the impression that the 1.5D corresponded to your every
    day "street" glasses. No I get the impression that the correspondence is to
    reading glasses. It is all too easy to be ambiguous.
    I still have difficulty understanding you.

    I think you are saying that your reference is that 1.5D reading glasses work
    well at 14inches = 0.356m. You then ask, "What is needed to work well at

    If you had a perfect eye, in the sense that it sees objects at infinity
    clearly without any accommodation from your eye, you would need a 1/0.356m =
    2.812D lens. Because you are using a 1.25D lens to get that effect, your eye
    already supplies the difference of 2.812-1.25 = 1.562D.

    To see clearly at 0.61m you need 1/0.61 = 1.639D if your eye were perfect.
    Because your eye is not perfect and supplies 1.562D, the lens has to supply
    only 1.639-1.562 = 0.077D. That is almost no lens at all.

    I would conclude that you need no lens at all to see a screen well at 0.61m.
    Alternatively, you have not given an accurate description of what is needed.


    -- Fermez le Bush--about two years to go.
    Salmon Egg, Mar 12, 2007
  15. grahamew

    grahamew Guest

    I am saying that 1.25D works for me at the usual reading distance,
    which I have assumed to be about 14", or .355m. I want another pair
    that works at 24", or .61m.

    I don't get your calculations. I do know that I get blurry vision and
    headaches while reading the screen at 24"/.61m. So, if 1.25D works for
    me at .355m, what will work for me at .61m? That is my question.
    grahamew, Mar 13, 2007
  16. grahamew

    Salmon Egg Guest

    I think your last paragraph quoted above says that you want the same thing I
    thought you wanted as in my second paragraph quoted above.

    I do not think that the presumption that your 1.25D provides clear vision at
    0.355m can be justified without actual testing. Certainly 1.5D at 0.355m
    would only work if your eye's accommodation provides the additional 1.562D I
    calculated above. (The high precision in my calculation is there just to
    prevent accumulation of roundoff error. Do not take it too seriously.)

    I was suggesting that you do not need glasses to view your screen at 0.61m.
    1.25 glasses will only blur your screen.

    -- Fermez le Bush--about two years to go.
    Salmon Egg, Mar 13, 2007
  17. That is correct. The other Bill's assumption that we would prescribe
    that which would make your eye not have to focus at all is incorrect.
    We only do that when there is zero accommodative amplitude or when there
    is a significant binocularity issue (esophoria) that demands it. In
    by far most cases, we only prescribe what is actually needed, and most
    patients will not accept such "full correction" as the Salmon Egg suggests.
    William Stacy, O.D., Mar 13, 2007
  18. Salmon Egg wrote:

    That would only be true if he had zero accommodation (an absolute
    presbyope), or had special binocularity issues. I think it's pretty
    obvious that neither of those are the case. If you mean he "needs" to
    supply 1.639 of total focusing convergence (which can be supplied by a
    combination of accommodation, depth of focus, and spectacle lens), then
    you are correct. Since most eyes have some accommodative response, and
    all have some depth of focus, we rarely prescribe the entire convergent
    demand for any distance, except for the occasional binocularity issue.
    Why not prescribe that "full" amount? It's not wearable for most
    people, because it is stronger than what is necessary and the stronger
    the spectacle power, the narrower the depth of focus becomes.
    William Stacy, O.D., Mar 13, 2007
  19. I can believe that he is comfortable supplying 1.5 accommodation at 14
    inches, for occasional reading at that distance, but is not comfortable
    supplying the entire 1.5 sustained focusing at computer distance without
    some additional "help". Not only do we usually prescribe less than the
    full convergent demand, but we also usually prescribe enough to take up
    some of the demand when appropriate. If he is emmetropic then the 1.00
    lens he calculated will not blur that computer monitor.

    btw I calculate 1.9 years remaining, and the accumulation of error is
    increasing daily.
    William Stacy, O.D., Mar 13, 2007
  20. grahamew

    grahamew Guest

    I was tested just over a year ago and that was the presecription I
    received. Looking back I should have had my eyes tested for the
    computer screen distance as well.
    They don't blur the screeen, they make every pixel stand out. In fact
    they are too strong, at least that's how it feels.
    grahamew, Mar 13, 2007
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.