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Medical Forum / General / Vision / August 2005Difference between 5" letters at 20' and 10" letters at 40'?
Is there a difference to the eye between 5" letters at 20', 10" letters at 40', 20" letters at 80', etc.? > Is there a difference to the eye between 5" letters at 20', 10" > letters at 40', 20" letters at 80', etc.? No. If you could remove all the peripheral and atmospheric cues, you couldn't tell the difference between 20', 40', 80' etc with just one eye. Those letters all subtend the same angle. -MT >No. If you could remove all the peripheral and atmospheric cues, you >couldn't tell the difference between 20', 40', 80' etc with just one eye. [quoted text clipped - 3 lines] >-MT > Mike , do you mean that it is same when a person uses 20/10 line at 10 feet for 20/20 and 20/20 line at 20 feet? What could be the peripheral and atmospheric cues? Lighting? John Dear LurfysMa, Subject: Eye-chart letter size, and resolution of the eye. Re: How this test was set up by Dr. Snellen Re: Is there a difference to the eye between 5" letters at 20', 10" letters at 40', 20" letters at 80', etc.? The intention of this "Standard" was to have eye's checked at 20 feet (or 6 meters). It was judged that 6 meters was far enough to be considered "optical infinity" for the eye. The human eye ( under good circumstances) can "resolve" to 1 minute-of-angle. Snellen made the letters 5 minute-of-angle in size, which makes the letters 0.9 cm at 6 meters. Keeping the angle constant, then this (20/20) letter would be 1.8 cm at 12 meters. Thus you are measureing the resolving power of the human eye by this method. For eas of use, some charts can be designed for 3 meters. (Small office.) Then, you could say 10/10 -- to be technically correct. Best, Otis Engineer -- > For eas of use, some charts can be designed for 3 meters. (Small > office.) Then, you could say 10/10 -- to be technically correct. In this case, a person with 1/3 diopter of myopia could not be distinguished from an emmetropic person. It is general consensus that 1/6 diopter is negligible, but perhaps 1/3 is not. I have been in small offices, and they sometimes use a mirror to fold the light path.  SignatureRobert T. Kopp http://analytic.tripod.com Dear Robert, Thanks for your input. The word "emmetropia" has several meaings, including refractive states running from -1/2 diotper to +3/4 diopter -- unless you specify that "emmetropia" is a refractive state of EXACTLY zero. The remark of the 10 foot (3 meter) is just the simple fact that some doctors use is for simple convenience. The "offical" chart remains at 20 feet, and refractive status to be measured with a trial-lens kit (or phoropter). Best, Otis > Mike , do you mean that it is same when a person uses 20/10 line at 10 > feet for 20/20 and 20/20 line at 20 feet? Yes, by similar triangles, those letters subtend the same angle. >What could be the peripheral and atmospheric cues? Lighting? The "atmospheric" cue is haze - how you know one mountain is further than another. "Peripheral" cues are all those other surrounding objects, relative motion, relative size - for instance, why the moon looks bigger when it's low in the sky. -MT > "Peripheral" cues are all those other surrounding > objects, relative motion, relative size - for instance, > why the moon looks bigger when it's low in the sky. !! I've never thought of that. I always assumed the moon looked larger low in the sky due a magnification effect of the atmosphere. Low in the horizon you are seeing it though more atmosphere than overhead. I'll have to measure the image next time. -Quick >"Peripheral" cues are all those other surrounding objects, relative motion, >relative size - for instance, why the moon looks bigger when it's low in the >sky. Although this explanation has merit, the primary effect is a physiological one. As one tilts his head back to observe the moon at the zenith, the lens of the eye is flattened by gravity and its magnifying power is decreased. (Thanks to the folks at sci.astro) Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical robopt@execpc.com ~~~~~~~~~~~~~~~~~~ "An expert is a person who has made all the mistakes that can be made in a very narrow field." - Niels Bohr Ummm, getting back to earth... If I have a 1 mm segment in my contact lens, what is the size of the image that would be covered at 30"? thanks, -Quick >> "Peripheral" cues are all those other surrounding >> objects, relative motion, relative size - for instance, [quoted text clipped - 16 lines] > "An expert is a person who has made all the mistakes that > can be made in a very narrow field." - Niels Bohr Using simple geometry and some fairly crude assumptions/estimations I get about 30 mm or 12 inches. That of course would be best case scenario, and assumes perfect (infinitely thin) ledges/lines. Your mileage will vary... w.stacy, o.d. >Ummm, getting back to earth... If I have a 1 mm segment >in my contact lens, what is the size of the image that would [quoted text clipped - 31 lines] > > Perfect! That should just about cover the viewable area of my 20" CRT... Although I guess you don't really look at that much at one time? Seems that if I look at a spot or word of text anything outside of about a 4" diameter circle is sort of "peripheral" to what you are looking at and you can't really tell if it's in focus or not. You have to shift your gaze past that. Surely this has been quantified/characterized? (should I have started a new thread? or is this still on topic for this one?) thanks, -Quick > Using simple geometry and some fairly crude assumptions/estimations I get about 30 mm or 12 inches. That of course would be best case scenario, and assumes perfect (infinitely thin) ledges/lines. Your mileage will vary... w.stacy, o.d. Quick wrote: Ummm, getting back to earth... If I have a 1 mm segment in my contact lens, what is the size of the image that would be covered at 30"? thanks, -Quick Robert Martellaro wrote: On Wed, 17 Aug 2005 13:52:19 GMT, "Mike Tyner" <mtyner@mindspring.com> wrote: "Peripheral" cues are all those other surrounding objects, relative motion, relative size - for instance, why the moon looks bigger when it's low in the sky. Although this explanation has merit, the primary effect is a physiological one. As one tilts his head back to observe the moon at the zenith, the lens of the eye is flattened by gravity and its magnifying power is decreased. (Thanks to the folks at sci.astro) Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical robopt@execpc.com ~~~~~~~~~~~~~~~~~~ "An expert is a person who has made all the mistakes that can be made in a very narrow field." - Niels Bohr > Perfect! That should just about cover the viewable area of my > 20" CRT... Although I guess you don't really look at that much [quoted text clipped - 8 lines] > on topic for this one?) > thanks, You could do that at any time (like I just did). I don't think trifocal contact lenses have been studied much. Good luck with them. Sounds like you're on the right track. Let us know how it works out, since you are probably a pioneer in this area. w.stacy, o.d. >> Perfect! That should just about cover the viewable area >> of my 20" CRT... Although I guess you don't really look [quoted text clipped - 17 lines] > track. Let us know how it works out, since you are > probably a pioneer in this area. doh... all those business terms come to mind. "opportunity", "interesting problem". I was starting to get clues though. "maybe only at handful have fitted those in the US" and when Fused Kontacts told me they didn't have a fitting kit for the trifocals but they did have very good consultants that would work directly with my OD to design the lenses. I'm looking forward to it. Seems there is a large hole with bifocals. One can go with multifocal simultaneous vision solutions which I've come to gather all have "compromises" that are significant. Sacrifice off of both ends or significantly off of one end or the other or resort to monovision. The flare, "soft" vision and other anomolies. I was a bit surprised this hasn't all been sorted out some time ago. I'm a slightly hyperopic (~+1.00) presbyope (~+2.00) who sits in front of a computer all day. I've always been very picky about my vision. I guess I could be in the worst case section of the fitting text book. The whole idea is to do without spectacles. I mean, that's what contacts are for right? I can do without correction just fine for distance although I do miss the sharpness of being able to see the tree line on a mountain ridge a few miles away. I want to get rid of the readers. If I have to insert and remove contacts depending on what I'm doing then we're back to readers. So my goal is perfect distance, perfect reading, and perfect computer (~30"). I'm willing to compromise at about 5 feet which is the distance between my nose and the golf ball on the tee since I can't golf like I used to anyway (and I've accepted that :)). Besides, once you're lined up you should be able to swing with your eyes closed. I think I can deal with adapting to moving my head more than my eyes most of the time. So it seems that all the stuff is aimed at broad range with "compromise" everywhere or 2 focal points, distance and some point closer. I would have thought 3 focal points to be the minimum requirement and more would have been done with trifocals already? -Quick > So it seems that all the stuff is aimed at broad range with > "compromise" everywhere or 2 focal points, distance > and some point closer. I would have thought 3 focal points > to be the minimum requirement and more would have > been done with trifocals already? As you probably know, spectacle trifocals are quite common and appreciated by many folks who have zero or close to zero accommodative amplitude, and for those 3 focal lengths is necessary/useful. The problem with trifocals that when either (or both) seg line infringes on the pupillary area, there is a certain amount of light scatter and/or double refraction at and around the line(s). Our theoretical clear zone of 12 inches with the 1 mm intermediate seg on the TS Tri would be well overlapped by interference from both distance and near zones, since the pupil size is probably on the order of 3 mm. Hopefully the "main" clear image through the intermediate seg would overshadow the other "focuses" to the extent that you won't have appreciable degradation of your vision. YMWV At least with the TS bi you can get the pupil fully covered by the distance area, with no interference from the seg when viewing distance objects, and vice-versa with near. What's probably kept me from trying the TS tri is that I'm pretty much guaranteed *some* interference in that mid seg. But however much interference there is from the lines themselves and from the other focal areas, I'm willing to bet that it will still give better vision than you'd get with ANY of the aspheric (smeared optics) alternatives, and maybe, just maybe it won't even be noticeable. That's why I'm interested in your input, as I always like to hedge my bets. Again, you'll be the first I ever encountered who has tried them, and I've been around a while (have fit a dozen or so TS Bis over the years). w.stacy, o.d. > Although this explanation has merit, the primary effect is a physiological > one. [quoted text clipped - 3 lines] > to > the folks at sci.astro) It took me a while to find that message. By the time I did, I had this nice set of arguments all scrounged up and now I see it was all tongue-in-cheek, and hugely funny, not to mention 15 friggin years old. Bad boy. :) <http://groups-beta.google.com/group/sci.physics/browse_thread/thread/ce6bff6635c f7cd1/8d47a56d4cd73c33?q=larger+eye+lens+moon&rnum=4&hl=en#8d47a56d4cd73c33> -MT >>"Peripheral" cues are all those other surrounding objects, relative >>motion, [quoted text clipped - 9 lines] > to > the folks at sci.astro) I think that is wrong. The size of the retinal image doesn't change, what does change its the mind's perception of the distance away. You can check this out by holding out a quarter at arm's length and using it to cover the moon, which is just does, both at the horizon and in the sky. The apparent size of the moon is different, the apparent size of the quarter doesn't change. One explanation I have heard: Near the horizon, the moon is perceived to be a long way away since it is behind trees, houses etc. In the sky, there are no clues to distance and the moon is assumed to be closer. The mind has a lot of experience with objects of similar retinal image size at different distances and knows that if two objects have the same retinal image size and one is further away, then the further away one is larger. This is basis of those funny rooms at science centres; the ones where you look in the window and people appear to shrink and expand as they walk through. The link below shows the effect: http://www.yorku.ca/eye/toc-sub.htm Dr Judy > Robert Martellaro > ~~~~~~~~~~~~~~~~~~ [quoted text clipped - 5 lines] > very narrow field." > - Niels Bohr >> eye is flattened by gravity and its magnifying power is decreased. >> (Thanks to the folks at sci.astro) > > I think that is wrong. The size of the retinal image doesn't change, what > does change its the mind's perception of the distance away. It was tongue-in-cheek. The article he referenced had a half-dozen explanations, all facetious and all funny. I thought of several rebuttals - f'rinstance presbyopes see the illusion too - but it wasn't a serious hypothesis. -MT >> Is there a difference to the eye between 5" letters at 20', 10" >> letters at 40', 20" letters at 80', etc.? [quoted text clipped - 3 lines] > >Those letters all subtend the same angle. That was my thinking, too, but it doesn't seem to work that way. I have been trying on contacts the past few weeks. I have been using reading glasses for 20 years, but never had a problem with distance. I swear I used to be able to read license plates a block away (I wish I had taken good measurements). I went in for an annual eye exam and the doctor told me I was borderline for passing a DMV eye test. I had noticed that distance vision was not as good, but I guess I hadn't realized how much it had degraded. Anyway, I went to see about contacts. The optician used the standard eye chart to determine which lenses were good for me. It seems to me that lenses that make the bottom line on the eye chart clear do not make it easer to read street signs and license plates at much longer distances. I've read here about focal length. Is that a factor? We tried monovision and that didn't work at all for me. I might not have given it a long enough trial, but I couldn't see well enough to drive safely. Close up vision was great. I now have distance-only contacts. They seem to help the most at intermediate range, but not much at long distance. The optician said that far-sighted people are the hardest to fit. They are used to being able to see gnats on mountain tops and they aren't happy with anything less. Any suggestions? I need to go in next week for another fitting. Dear LurfysaMa, Just for the fun of it you might enjoy checking your DMV-Snellen at: http://www.smbs.buffalo.edu/oph/ped/IVAC/IVAC.html Just type 1 inch for line-length, and 10 feet for distance and see what you can read. The DMV generallly requires 20/40 or better in most states. If the "contacts" bother you, and you pass the DMV, you could go back to using (plus) reading lenses. Best, Otis (Engineer) > Just for the fun of it you might enjoy checking your DMV-Snellen at: > http://www.smbs.buffalo.edu/oph/ped/IVAC/IVAC.html > > Just type 1 inch for line-length, and 10 feet for distance > and see what you can read. Otis, For some reason (OK, the design) they have a horizontal split in the screen that make the screen difficult to scroll. Don W Dear Don, Subject: Snellen Chart for Your Monitor. The "Snellen" does not "scroll". Just click on "Display" to get a new set of random letters. You can change the size of the letters by clicking on "Larger" and "Smaller". Easy! Best, Otis >>> Is there a difference to the eye between 5" letters at 20', 10" >>> letters at 40', 20" letters at 80', etc.? [quoted text clipped - 21 lines] > make it easer to read street signs and license plates at much longer > distances. For those tasks, whether you see subjectively better will depend upon the distance and the size of letter on the street sign or license plates. Remember, that the eye cannot see better than about 20/20 which is about a 1 cm letter at 20 ft, 10 cm letter at 200 ft and so on. If you are trying to see a 5 cm letter (license plate) at 200 ft, you won't be able to, no matter how good your glasses are. Also, clarity in the real world is affected by contrast (black on white is better, gray on gray or colour on colour is worse), lighting (if dim, then you won't get 20/20), atmospheric haze and so on. Dr Judy > I've read here about focal length. Is that a factor? > [quoted text clipped - 10 lines] > > Any suggestions? I need to go in next week for another fitting. |
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