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Medical Forum / General / Vision / February 2008Change in lens power with distance
I wonder if someone could help me understand something about glasses. I am 62, myopic, and don't think I have any accommodation to speak of. My prescription is about -2,-2.25, averaging in the astigmatism. When I got my eyes checked, I mentioned that I wanted a special pair of glasses to watch TV, at a distance of 11 ft. (3.35 m), because I found my old glasses blurry at that distance. I was told that the distance of the eye chart was 11 feet, and that was taken as equivalent to infinity. My new glasses seem ok at 11 ft. for watching TV, but not as good outdoors at infinity. Is it true that going from infinity to 3.5 m, to maintain perfect focus, the negative power of the glasses would have to be reduced by 1/3.5 m, or about 0.3 diopters? If true, wouldn't this be a significant difference in sharpness?  SignatureJohn > I wonder if someone could help me understand something about glasses. > I am 62, myopic, and don't think I have any accommodation to speak of. [quoted text clipped - 5 lines] > equivalent to infinity. My new glasses seem ok at 11 ft. for watching > TV, but not as good outdoors at infinity. In my experience in the US (I am not a doctor, and have nothing to do with vision care, other than I like to see), vision is tested at 20 feet, and that is assumed to be roughly the same as infinity. Sharpness has a lot to do with light level. Testing in the OD office is done under controlled levels of light. If your glasses are actually set to 11 feet, then you should see fine at infinity when there is enough light. > Is it true that going from infinity to 3.5 m, to maintain perfect > focus, the negative power of the glasses would have to be reduced by > 1/3.5 m, or about 0.3 diopters? If true, wouldn't this be a > significant difference in sharpness? Perfect focus is rather elusive. Optometrists in the US generally test in increments of .25D. Depending on what kind of astigmatism you have, that might have an effect also. If you are bothered by your vision at infinity, it's worth a trip to your doctor. Many people wouldn't want to be bothered with switching glasses for such a small difference, but if it bothers you, and you are willing to wear different glasses for tv and infinity, then it's worth talking to your doctor and seeing how that would work. SignatureDan Abel Petaluma, California USA dabel@sonic.net Dear John, A lot of the "difference" depends on how the minus is prescribed. You are probably correct that your accommodation (stop-to-stop) is limited, but you do have "depth of field". In bright light, that can be from 1 to 1.5 diopters, depending on aperture. But I would talk to your OD about this issue. He can give you slightly stronger minus lenses if you want that additional sharpness of vision. A am certain he will be willing to accommodate you (no pun intended). Otis > I wonder if someone could help me understand something about glasses. > I am 62, myopic, and don't think I have any accommodation to speak of. [quoted text clipped - 11 lines] > -- > John > I wonder if someone could help me understand something about glasses. > I am 62, myopic, and don't think I have any accommodation to speak of. [quoted text clipped - 9 lines] > 1/3.5 m, or about 0.3 diopters? If true, wouldn't this be a > significant difference in sharpness? The short answer is that your calculation is correct. It should not depend upon your astigmatism. I am not a vision professional, but I do understand optics. I believe that lens power ordinarily is available in 0.25D steps. There may be finer steps available. Bill >I wonder if someone could help me understand something about glasses. >I am 62, myopic, and don't think I have any accommodation to speak of. >My prescription is about -2,-2.25, averaging in the astigmatism. >When I got my eyes checked, I mentioned that I wanted a special pair >of glasses to watch TV, at a distance of 11 ft. (3.35 m), >because I found my old glasses blurry at that distance. That's a common complaint when the Rx changes towards less minus or more plus. Has it changed? Are your eyes healthy? Post the new unmodified (distance) Rx and the lens powers of the old glasses. Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa Wi. ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman >>I wonder if someone could help me understand something about glasses. >>I am 62, myopic, and don't think I have any accommodation to speak of. [quoted text clipped - 7 lines] >Has it changed? Are your eyes healthy? Post the new unmodified (distance) Rx and >the lens powers of the old glasses. I had (2004): -2.5 -.5 90 -3.0 -.5 50 add +2.25 I now have (2008) -1.75 -.5 90 -2.25 -.5 50 add +2.25 It's changed a lot. It may be worth saying that the older prescription was obtained by an automatic instrument, while the new one was obtained by a phoropter and "is this better, or is this better"? How much variation will there be between these two methods? I have had one pair of glasses made to the new script, and the bifocals are almost unusable, because the lower segment has no remaining correction at all, which may be ok for reading, but I don't need glasses for reading. What I do need is some way to see the speedometer of my car clearly, and to see items on grocery store shelves or on a table in front of me without removing my glasses. I guess that's another matter, along with the fact that the bifocal line lies almost within my level-headed field of vision. Anyhow, referring to the original problem, the old glasses were ok for driving, but not sharp and clear for watching TV, while the new ones are good for watching TV but not sharp and clear for driving. I'd like to order a new pair of bifocals that will be sharpest for driving. At this point, I'm thinking of going with -2, -2.5 (restoring one quarter diopter of convergence), with add of 1.5, which would make the bifocal segment equal to my current computer glass prescription, which is -.5, -1, and which works fine. SignatureJohn >>>I wonder if someone could help me understand something about glasses. >>>I am 62, myopic, and don't think I have any accommodation to speak of. [quoted text clipped - 7 lines] >>Has it changed? Are your eyes healthy? Post the new unmodified (distance) Rx and >>the lens powers of the old glasses. >I had (2004): >-2.5 -.5 90 [quoted text clipped - 7 lines] > >add +2.25 Ok, that explains why the TV is blurry with the old glasses. > It's changed a lot. It may be worth saying that the older >prescription was obtained by an automatic instrument, while the new >one was obtained by a phoropter and "is this better, or is this >better"? How much variation will there be between these two methods? Autorefractors and retinoscopy are starting points, but the final Rx is determined with lenses/phoropter. > I have had one pair of glasses made to the new script, and the >bifocals are almost unusable, because the lower segment has no >remaining correction at all, which may be ok for reading, but I don't >need glasses for reading. They're not unusable, you just don't like them. >What I do need is some way to see the >speedometer of my car clearly, and to see items on grocery store >shelves or on a table in front of me without removing my glasses. You and millions of other presbyopic ametropes. >I guess that's another matter, along with the fact that the bifocal line >lies almost within my level-headed field of vision. Consider progressive power lenses. Bias the design for distance and intermediate. Take them off to read for extended periods. You still may need computer glasses. > Anyhow, referring to the original problem, the old glasses were ok >for driving, but not sharp and clear for watching TV, They worked poorly for the TV distance because the Rx was outdated and incorrect. >while the new >ones are good for watching TV but not sharp and clear for driving. That's probably the doctors fault because the correct Rx would have made both distances ok. However, I wasn't there, and you may have insisted that the doc give you an Rx that was optimized for the TV distance. >I'd >like to order a new pair of bifocals that will be sharpest for >driving. At this point, I'm thinking of going with -2, -2.5 (restoring >one quarter diopter of convergence), with add of 1.5, which would make >the bifocal segment equal to my current computer glass prescription, >which is -.5, -1, and which works fine. Look, you need to get good advice from both the doctor and optician. If you're not comfortable with your present eye care providers then shop for new ones. It may help if either the doctor or optician is about your age. Stick with the independents, explain the circumstances and your concerns, and if you get blank stares then move on. Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa Wi. ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman
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