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Medical Forum / General / Vision / June 2006eye glass question: how does Y diopters map to 20/XXX
If an eye requires a spherical correction of -4.00 diopters, what distance does that map to (XXX) as compared to what a person with perfect eyes can see at 20 feet? For example, is it 20/200? 20/300? Is there a formula for calculating 20/XXX based on Y diopters? Dear Frank, There is no exact "formula". But a rough "calculation" suggest that it takes about -1 diopter to clear the 20/20 line if a person has naked-eye visual-acuity of 20/70. So, using you example, -4 * 70 = 20/280 visual acuity. Of course, you could have the person read the chart at 20/280, and using a trial-lens kit, find the weakest minus that would clear the 20/20 line. Hope this clarifies this issue for you. Best, Otis > If an eye requires a spherical correction of -4.00 diopters, what > distance does that map to (XXX) as compared to what a person with [quoted text clipped - 3 lines] > > Is there a formula for calculating 20/XXX based on Y diopters? > and using a trial-lens kit, find the weakest > minus that would clear the 20/20 line. Why would he want blurry vision? "O'Leary and his colleagues at the National University of Malaysia in Kuala Lumpur, undercorrected the sight of half the children and fully corrected the rest. Then they measured the length of the eyeball with ultrasound every six months. To their surprise, they found that the eyeball elongates faster when vision is undercorrected." -New Scientist, Nov 2002 > Dear Frank, > > There is no exact "formula". Dear Frank, Please disregard Otis's postings. He is not in the medical profession and not in any position to give medical advice. Thanks! Allen > > Dear Frank, > > [quoted text clipped - 8 lines] > > Allen I've always thought that there is no formula for that. I have a friend with -2 who sees 20\200 uncorrected... he's not -4. Strange. Dear Dragon, I said APPROXIMATE. I said NO FORMULA. But, the correct method would be to use a trial lens FOR THAT PERSON. Thus there can be no formula. Best, Otis > > > Dear Frank, > > > [quoted text clipped - 12 lines] > I have a friend with -2 who sees 20\200 uncorrected... he's not -4. > Strange. Dear Dragon, Subject: off the meds again. Obviously, I have stopped taking my medication because of potential complex interactions with the "ECT." Please be gentle with me in the coming weeks. I may tend toward incoherence, irrationality, and the random spewing of nonsense. Best, Odis At Least I'm Finally Outpatient > Dear Dragon, > > I said APPROXIMATE. Dear Dragon, Please disregard Otis's postings. He is not in the medical profession nor in any position to give medical advice. Thanks! Allen > If an eye requires a spherical correction of -4.00 diopters, what > distance does that map to (XXX) as compared to what a person with [quoted text clipped - 3 lines] > > Is there a formula for calculating 20/XXX based on Y diopters? There is a formula, but it has problems. IF you mean just myopes with no astigmatism, it's still a scattered plot and scattered plots are notoriously unreliable. For a best-fit formula, a reasonable attempt would be 0.43 x D = log(20/xxx) That's from a 1997 thread here on s.m.v. Someone with -400 diopters of pure myopia isn't likely to see better than 20/200, in my exam room, without squinting. With squinting, or bright light, they might see 20/40 or better, and 20/25 through a pinhole. -MT, OD There are actual, many variables such as lighing and how large or small the pupil is at the time of testing. The smaller the pupil, the greater the clarity to a point due to increasing depth of field. --Dr. Tom |
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