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Medical Forum / General / Vision / March 2007Glasses for Computer use.
Hi, 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. Graminator > Hi, > I'm blowing in to ask if anyone can advise me. A year ago I got my [quoted text clipped - 13 lines] > > Graminator 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. > Hi, > I'm blowing in to ask if anyone can advise me. A year ago I got my [quoted text clipped - 13 lines] > > Graminator 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. On 3/9/07 1:21 PM, in article 1173475285.913967.307350@j27g2000cwj.googlegroups.com, "grahamew@myway.com" > Hi, > I'm blowing in to ask if anyone can advise me. A year ago I got my [quoted text clipped - 13 lines] > > Graminator I want to emphasize that I am not a medical person, but I do understand optics. Presuming your actual prescription is currently 1.25D as has been suggested by previous posters, your glasses should (approximately) make an object at a distance of 0.8 meter look as if it were at infinity. What you want is for your computer screen at 0.61m to look as if it were at infinity. Someone with emmetropic ("normal") vision requires 1/0.61 or 1.64D to do that. You require 1.15D to get normal vision so you need glasses with the sum of the two powers or a total of 2.89D. You should be able to find over the counter reading glasses with 3.0D. I get the cheapest ones I can find for about a dollar a pair. There may be same second order modification to take care of centering or the finite separation between your eye and the lens, but I will leave that to the optometrists who know the gory details. Bill -- Fermez le Bush--about two years to go. > I want to emphasize that I am not a medical person, but I do understand > optics. Presuming your actual prescription is currently 1.25D as has been > suggested by previous posters, your glasses should (approximately) make an > object at a distance of 0.8 meter look as if it were at infinity. What you > want is for your computer screen at 0.61m to look as if it were at infinity. 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. DrG On 3/10/07 5:54 AM, in article 1173534895.551926.311440@q40g2000cwq.googlegroups.com, "Dr. Leukoma" <drg@leukoma.com> wrote: >> I want to emphasize that I am not a medical person, but I do understand >> optics. Presuming your actual prescription is currently 1.25D as has been [quoted text clipped - 7 lines] > > DrG Does not what I suggested above do just what you say needs to be done? Bill -- Fermez le Bush--about two years to go. > On 3/10/07 5:54 AM, in article > 1173534895.551926.311...@q40g2000cwq.googlegroups.com, "Dr. Leukoma" [quoted text clipped - 17 lines] > Bill > -- Fermez le Bush--about two years to go. 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. DrG On 3/10/07 8:30 PM, in article 1173583799.952071.7520@8g2000cwh.googlegroups.com, "Dr. Leukoma" <drg@leukoma.com> wrote: >> On 3/10/07 5:54 AM, in article >> 1173534895.551926.311...@q40g2000cwq.googlegroups.com, "Dr. Leukoma" [quoted text clipped - 7 lines] >>>> want is for your computer screen at 0.61m to look as if it were at >>>> infinity. <snip> > Yes, Bill, but the devil is in the details. The conditions at > infinity are simply not the same for everybody, i.e. nearsighted folks [quoted text clipped - 3 lines] > > DrG 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. Bill -- Fermez le Bush--about two years to go. > 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. 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. DrG On 3/11/07 11:40 AM, in article 1173638405.436177.246900@q40g2000cwq.googlegroups.com, "Dr. Leukoma" <drg@leukoma.com> wrote: > 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. 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? Bill -- Fermez le Bush--about two years to go. > 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? 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? DrG > > 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 [quoted text clipped - 12 lines] > > DrG 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 following: - 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 (. 61-.355=.255) - 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. >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. That's because he added instead of subtracted. I think one of our orbiters made a new crater on Mars for similar reasons. Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa Wi. ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman On 3/12/07 11:15 AM, in article 1173723313.231100.42710@64g2000cwx.googlegroups.com, "grahamew@myway.com" > 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. 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. > William I took your formula, and with a bit of algebra I deduced the > following: [quoted text clipped - 19 lines] > et al. If I get +0.75 I would have to buy online cause I haven't seen > any that power on the shelf. 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 0.61m?" 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. Bill -- Fermez le Bush--about two years to go. > On 3/12/07 11:15 AM, in article > 1173723313.231100.42...@64g2000cwx.googlegroups.com, "graha...@myway.com" [quoted text clipped - 56 lines] > > -- Fermez le Bush--about two years to go. 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. On 3/12/07 6:47 PM, in article 1173750439.154602.172640@j27g2000cwj.googlegroups.com, "grahamew@myway.com" >> I still have difficulty understanding you. >> [quoted text clipped - 25 lines] > 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. 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. Bill -- Fermez le Bush--about two years to go. > 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 [quoted text clipped - 4 lines] > I was suggesting that you do not need glasses to view your screen at 0.61m. > 1.25 glasses will only blur your screen. 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. > On 3/12/07 6:47 PM, in article > 1173750439.154602.172...@j27g2000cwj.googlegroups.com, "graha...@myway.com" [quoted text clipped - 35 lines] > I do not think that the presumption that your 1.25D provides clear vision at > 0.355m can be justified without actual testing. 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. > Certainly 1.5D at 0.355m > would only work if your eye's accommodation provides the additional 1.562D I [quoted text clipped - 3 lines] > I was suggesting that you do not need glasses to view your screen at 0.61m. > 1.25 glasses will only blur your screen. 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. > Bill > -- Fermez le Bush--about two years to go.- Hide quoted text - > > - Show quoted text - > To see clearly at 0.61m you need 1/0.61 = 1.639D if your eye were perfect. 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. >>>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 [quoted text clipped - 41 lines] > et al. If I get +0.75 I would have to buy online cause I haven't seen > any that power on the shelf. 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. On 3/12/07 5:51 AM, in article 1173703887.214715.277550@p10g2000cwp.googlegroups.com, "Dr. Leukoma" <drg@leukoma.com> wrote: >> 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 [quoted text clipped - 12 lines] > > DrG 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 correctly. Bill -- Fermez le Bush--about two years to go. > 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. 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. DrG |
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