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Medical Forum / General / Vision / August 2006Snellen Chart and Minus Diopter
Does anyone knows how visual acuity measured with the Snellen Chart corresponds to a minus diopter? For instance, what is the diopter for 20/70? Thanks. > Does anyone knows how visual acuity measured with the Snellen Chart > corresponds to a minus diopter? > > For instance, what is the diopter for 20/70? > > Thanks. I don't think it works that way. You could be extremely farsighted, or astigmatic, or whatever. Maybe there are general rules though, assuming pure myopia without astigmatism. Someone else can answer. -- > Does anyone knows how visual acuity measured with the Snellen Chart > corresponds to a minus diopter? > > For instance, what is the diopter for 20/70? > > Thanks. Unaided acuity is a poor predictor of refractive error; too many other factors influence unaided acuity. As a rough guess, if myopia is known to be the cause of the reduced acuity and if there is not also astigmatism present, then unaided acuity of 20/70 would correspond to anywhere from -0.75D to -2.50D of myopia. Dr Judy > Does anyone knows how visual acuity measured with the Snellen Chart > corresponds to a minus diopter? > > For instance, what is the diopter for 20/70? > > Thanks. I do. I have been researching this for some time and have seen the results of many people, almost always myopes. I have even made a formula that attempts to show how blurry youd see from an objective point! The math is based on the blur circle on the retina caused by defocus in diopters. Someone at 20/70 is likley to be -1.5 diopters myopic. Take spherical equivalent if astigmastim is in the mix. -.25 diopters results in minimal blur and doesnt cost a single line. Sometimes half a line at most. Only malingering people make a deal out of this. -.5 diopters results in a reduction of a single snellen line. If 20/20, then your UCVA would be 20/25. Correction is not needed. -1 diopters is the smallest amount of blur that people start to complain about and the lowest prescription that justifies very low dependance on glasses, such as for driving. Normal people go most of the time without correction. UCVA generally is 20/40 to 20/50 -2 diopters blurs enough for part time glasses wear. Can easily go without glasses around the house and for intermediate distance. UCVA is 20/100 to 20/150. Alot of websites say its 20/200 but I disagree. One problem is many snellen charts have nothing between 20/100 and 20/200 so someone could be just shy of 20/100, yet be put down as 20/200. -3 diopters starts getting bad. My research shows it to be 20/200 to 20/300, but some websites say its 20/400 but I disagree on it being this bad. -.75 diopters will NOT blur to 20/70 in a normal healthy eye. Many people see 20/30 with such a low minus and dont need correction. -2.5 diopters will almost always blur more than 20/70 unless you have 20/10 BCVA or you cheat and squint. Great thread! Im sure the optometrists here can share their experience on the average snellen score for a given myopic prescription of -.5, -1. -1.5, -2, etc. Don't listen to ace. And please never drive or go hunting with him =8^O He has a very high tolerance for bad vision. Go to the drug store and put on some +1 readers. Tell me how long you'd tolerate that for anything besides reading. That's ace's threshold for bothering with glasses. > > Does anyone knows how visual acuity measured with the Snellen Chart > > corresponds to a minus diopter? [quoted text clipped - 44 lines] > on the average snellen score for a given myopic prescription of -.5, > -1. -1.5, -2, etc. -- > Don't listen to ace. And please never drive or go hunting with him > =8^O [quoted text clipped - 3 lines] > anything besides reading. That's ace's threshold for bothering with > glasses. I disagree with Charles, hes super, super picky about his vision and probably picky about everything else in life like the perfectionist he is. I provided useful information about diopters to 20/x. Charles is only partially correct about my tolerance for bad vision. Myopia is NOT bad vision, I have friends that are -1 to -2 who see relatively well in the distance without glasses and they see much_better_than_emmetropes from near. Trying on readers to simulate myopia isnt quite the same because youll be bothered more by the scratches, smear, magnification, glare, etc from the lenses. Thats the reason very low myopes usually forgo glasses, they couldnt be bothered with the hassles for such a small difference in clarity at the expense of quality. Someone whos -1 and 20/40 without glasses is happier than someone dealing with the hassles of glasses. Ive been asked why I wont get lasik if im not picky about vision. The reason is because lasik affects quality of vision and also causes all kinds of problems glasses cant fix. Natural vision improvement and orthoK is where its at. I was reading this post and I was wondering if astigmatism would make it easier or harder on the Snellen chart. If you were(for example)-4.00 with no astigmatism versus -4.00 with a +3.00 astigmatism who would be more likely to do better on the chart. >> Don't listen to ace. And please never drive or go hunting with him >> =8^O [quoted text clipped - 18 lines] >also causes all kinds of problems glasses cant fix. Natural vision >improvement and orthoK is where its at. > I was reading this post and I was wondering if astigmatism would make it > easier or harder on the Snellen chart. If you were(for example)-4.00 with no > astigmatism versus -4.00 with a +3.00 astigmatism who would be more likely to > do better on the chart. -4 +3 cylindar should be rewritten as -1 -3. Thats a spherical equivalent of -2.5 so he would see much better than a -4 myope. However its much better to be -4 without astigmatism because you will be able to read from 10" without glasses. Also you can get orthoK and improve your -4 >I was reading this post and I was wondering if astigmatism would make it > easier or harder on the Snellen chart. If you were(for example)-4.00 with > no > astigmatism versus -4.00 with a +3.00 astigmatism who would be more likely > to > do better on the chart. The +3.00 means the steepest and flattest aspects of the football-shaped surface are 3.00 diopters apart - one is -4.00 and the other is -1.00 (as in -4 +3 =-1) . So depending on the orientation, the eye could be -1.00 for vertical lines and -4.00 for horizontal lines, or vice versa. Orientation is critical because someone with this prescription might miss the big "E" on the Snellen chart but might make out "W" or "M" in much smaller print. So the -1.00 aspect _does_ improve overall acuity for the person with astigmatism. Acuity might be roughly what you'd expect for someone with a spherical cornea (no astigmatism) at the geometric average of the flattest and steepest curves. The geometric average is called the "spherical equivalent." In "plus" notation we calculate the steepest curve (-4.00) plus half the difference (3.00/2 = 1.50) and the result is a spherical equivalent of -2.50 (= -4.00 +1.50). "Plus" notation is when prescriptions are written -4.00 +3.00 x 090. In "minus" notation the same prescription would be -1.00 -3.00 x 180. In minus notation, start with the _flattest_ curve and _subtract_ half the difference, and the spherical equivalent works out the same in either notation. -MT, OD bump for Fidelis K to read and others. Keep it comming, many others also wonder the correlation of myopic diopters to 20/x! |
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