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Medical Forum / General / Vision / December 2006correcting vision better than 20/20
Can vision be safely corrected better than 20/20? I'm guessing there must be tradeoffs of correcting vision with glasses, contacts, or laser beyond 20/20. Dear Bucky, Subject: The "standard" 20/20. Most people (no medical condition -- and young), will have visual resolution of 1 minute-of-angle, or be able to read 3/8 inch letters at 20 feet. A number of people can have vision sharper than 20/20, with the use of a stronger minus lens. Some ODs use this as a standard to prescribing a minus lens, and this is called prescribing for "Best Visual Acuity". Most DMV tests require 20/40 with both eyes, or 20/40 in the better eye, with no lens. > Can vision be safely corrected better than 20/20? Otis> That is a "loaded" question. It depends on exactly what you mean by "safely". Ther are those who suggest that a strong minus lens accellerates the development of nearsightedness. The "jury" is still out on that subject. Some ODs call the minus, "poision glasses for children." I'm guessing there > must be tradeoffs of correcting vision with glasses, contacts, or laser > beyond 20/20. Otis> That is a judgment you will have to make for yourself. Best, Otis > A number of people can have vision sharper than > 20/20, with the use of a stronger minus lens. Except for those who see better with a stronger _plus_ lens, or a stronger _cylinder_ lens. The majority of people in the US are not nearsighted. > Some ODs use this as a standard to prescribing > a minus lens, and this is called prescribing > for "Best Visual Acuity". Of course hyperopes have no BVA, and it's only the OD's who use this standard. > Most DMV tests require 20/40 with both > eyes, or 20/40 in the better eye, with no > lens. Of course nobody needs to see better than 20/40. Anything better than 20/40 is harmful to the eye. You might as well killfile me again, Mark. I see clouds puckerin' up to come a-squirt. -MT > Dear Bucky, > [quoted text clipped - 33 lines] > > Otis This is all very interesting, Otis. WITHOUT pointing to any point of your own anatomy, could you kindly cite your source for this information? Thanks. > A number of people can have vision sharper than > 20/20, with the use of a stronger minus lens. As Regan once said, "There you go again!". In fact, the vast majority of humans can see better than 20/20. Some do so with a little plus. Some with a lot of plus. Some with some cyl. Some with a zero power rigid contact lens. Some with no lens at all (in fact MOST humans in the world fall into this category). And yes, some do so with minus lenses. Some with WEAKER minus lenses, some with moderate power minus, and yes, a few do so with strong power minus. So what? Get over it. w.stacy, o.d. > That is a "loaded" question. It depends on exactly > what you mean by "safely". Ther are those who > suggest that a strong minus lens accellerates > the development of nearsightedness. OK, I'm not talking about long term safety. I just mean "safely" meaning you can wear that correction all day without getting a headache. Can a typical nearsighted person be corrected to 20/5 or 20/1, or what's the limit? > Can vision be safely corrected better than 20/20? I'm guessing there > must be tradeoffs of correcting vision with glasses, contacts, or laser > beyond 20/20. 20/20 simply means that you see at 20 feet what a "normal" person sees at 20 feet. The goal in correcting vision is to give you the best possible, not some arbitrary number.  SignatureDan Abel dabel@sonic.net Petaluma, California, USA > The goal in correcting vision is to give you the best possible, not some > arbitrary number. So why not attempt to correct nearsighted people's vision to 20/5 or 20/1? Isn't that better than 20/20? > So why not attempt to correct nearsighted people's vision to 20/5 or > 20/1? Isn't that better than 20/20? Because there's a limit to how many photoreceptors you have in your retina. If you focus a camera perfectly - not behind the object, not in front of the object, but exactly on the object, then the film is what limits the resolution of the photo. -MT > If you focus a camera perfectly - not behind the object, not in front of the > object, but exactly on the object, then the film is what limits the > resolution of the photo. Ahh, thank you, that was a helpful analogy. How about lenses that also magnify then? Let's say a baseball player who is 20/20 wants to be 20/10. Could he get lenses that magnify to achieve that resolution? In this case, I suppose that the tradeoff would be field of view. But is this possible? > How about lenses that also magnify then? Let's say a baseball player > who is 20/20 wants to be 20/10. Could he get lenses that magnify to > achieve that resolution? In this case, I suppose that the tradeoff > would be field of view. But is this possible? Of course it's possible, but it may not be practical. If the baseball player is _farsighted_, it's a simple matter of pushing the glasses out far enough off his nose to magnify, as you would with a "magnifying" glass (both are _plus_ lenses.) We call this increasing the "vertex distance", the distance from the cornea to the back lens surface. The problem with doing this is 1) you may have to push them too far out to be practical, and 2) the lens _power_ changes with increases in vertex distance so the actual prescription must be changed to get the same lens "effectivity." OTOH, if the baseball player is _nearsighted_, his normal lenses _reduce_ image size, so there's no manipulation that will make the image _bigger_. Best you could do is contacts, where the vertex distance is zero, and no minification occurs. Both of those options assume you're limited to a single pair of lenses, one in front of each eye. If you really want mag, you make a telescope with two lenses over each eye: a plus lens placed forward to magnify, and a minus lens placed closer to the eye to correct the focusing error introduced by the plus. With the minus lenses being closer, they won't minify much, while at greater distance the plus lenses _will_ magnify and the system is designed to create a net zero power (or net -1.00 or +3.00, whatever the original prescription might have been.) -MT Dear Bucky, Bucky> How about lenses that also magnify then? Let's say a baseball player > who is 20/20 wants to be 20/10. Could he get lenses that magnify to > achieve that resolution? In this case, I suppose that the tradeoff > would be field of view. But is this possible? Yes, a 4 power telescope will provide that type of angular magnification -- but not very practical. Otis ============= > > If you focus a camera perfectly - not behind the object, not in front of the > > object, but exactly on the object, then the film is what limits the [quoted text clipped - 6 lines] > achieve that resolution? In this case, I suppose that the tradeoff > would be field of view. But is this possible? > Dear Bucky, Dear Bucky, Please disregard Otis's postings. He is not in the medical profession and not in any position to give medical advice. Thanks! Allen > Dear Bucky, > [quoted text clipped - 9 lines] > > Otis Ok otis, now you amaze me. You actually made a statement that makes sense. I'm dumbfounded. I don't know what to say. If you are that smart, why then do you keep posting the other nonsense stuff on this newsgroup? w.stacy, o.d. Ok otis, now you amaze me. You actually made a statement that makes sense. I'm dumbfounded. I don't know what to say. If you are that smart, why then do you keep posting the other nonsense stuff on this newsgroup? Otis> Thanks! Otis> And as you might know, I do have friends in both ophthamology and optometry. But we respect each other. They have been wonderful. Best, Otis w.stacy, o.d. > > Dear Bucky, > > [quoted text clipped - 16 lines] > > w.stacy, o.d. >> If you focus a camera perfectly - not behind the object, not in front of >> the [quoted text clipped - 7 lines] > achieve that resolution? In this case, I suppose that the tradeoff > would be field of view. But is this possible? In other words, could a baseball player play well wearing binoculars? <g> >> The goal in correcting vision is to give you the best possible, not some >> arbitrary number. > >So why not attempt to correct nearsighted people's vision to 20/5 or >20/1? Isn't that better than 20/20? There are two factors affecting visual acuity - optical aberrations and retinal density. 20/20 is what's expected of a person with baseline optical aberrations and normal retinal density. I don't know off hand what a normal retina could do with the best possible correction of optical aberrations (which cannot be removed entirely - perfect optics cannot exist, due to the wavelike properties of light), but it certainly won't be 20/5, let alone 20/1. If you want that kind of "correction", you want a bionic eye. Signature- Mike Ignore the Python in me to send e-mail. > > The goal in correcting vision is to give you the best possible, not some > > arbitrary number. > > So why not attempt to correct nearsighted people's vision to 20/5 or > 20/1? Isn't that better than 20/20? Yes it is, but the AVERAGE of a population was measured and established to be about 1 minute-of-arc resolution. Or the AVERAGE person could be "corrected" to 20/20, or reading 0.9 cm letters at 6 meters. Obviously, some can do better, and some can not read this size letters. In recognition of this fact, the DMV departments had set their standard as reading 1.8 cm letters at 6 meters. Because some people can not even be "corrected" to 20/20. Best, Otis >> The goal in correcting vision is to give you the best possible, not some >> arbitrary number. > > So why not attempt to correct nearsighted people's vision to 20/5 or > 20/1? Isn't that better than 20/20? In my experience when the lens was made stronger (beyond optimal) the acuity didn't improve. I just experienced discomfort and realized I needed a better optometrist. To those that may care recent research has determined that if use the photoreceptor density in the fovea, part of the eye that gives most detailed vision in normal eyes, the estimate for acuity is around 20/8, believe it or not. Now that is not the entire story, the light gets scattered/reflected/lost a little bit while traveling through the eye detracting from this 20/8 possibility. I can see 20/15+ with both eyes, corrected with toric soft contact lenses, anything less and I go nuts. However, not everyone needs/appreciates better acuity, but most are happy with 20/20. The reason for the 20/20 can have a lot to due with leading an exam, if 20/20 acuity or better is met you can rule out a lot of pathology or additional problem, if 20/20 is not met investigation as to why is necessary. Sorry to be long winded. ODTOBE > >> The goal in correcting vision is to give you the best possible, not some > >> arbitrary number. [quoted text clipped - 5 lines] > didn't improve. I just experienced discomfort and realized I needed a > better optometrist. > I can see 20/15+ with both eyes, corrected with toric soft contact > lenses, anything less and I go nuts. However, not everyone > needs/appreciates better acuity, but most are happy with 20/20. They better be happy. By definition, 20/20 is "normal" vision. SignatureDan Abel dabel@sonic.net Petaluma, California, USA > However, not everyone >needs/appreciates better acuity, For absolute proof of that, just look at people's eyeglasses -- how clean/dirty they are! (Likewise, check out their cars' windshields -- same thing!) David |
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