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Medical Forum / General / Vision / March 2006Telescopes and low vision.
If one is looking at a certain visual scene, say with a 7x binocular. And then one physically moves 7x closer. The resulting image may not appear as bright (or clear) when one is at a distance. Why is this? Don W. probably because you are focusing the binocular oculars to correct an existing refractive error. w.stacy, o.d. > If one is looking at a certain visual scene, say with a 7x binocular. And > then one physically moves 7x closer. The resulting image may not appear as > bright (or clear) when one is at a distance. Why is this? > > Don W. On 3/10/06 5:33 PM, in article ENpQf.74894$PL5.20791@newssvr11.news.prodigy.com, "William Stacy" <wstacy@obase.net> wrote: > probably because you are focusing the binocular oculars to correct an > existing refractive error. [quoted text clipped - 6 lines] >> >> Don W. There is a good reason for the opposite to be tru and is easy to observe. Atmospheric turbulence can really screw up what you see (bad seeing). It is most easily observed on a hot sunny day when looking across a parking lot. Bill -- Ferme le Bush > There is a good reason for the opposite to be tru and is easy to observe. > Atmospheric turbulence can really screw up what you see (bad seeing). It [quoted text clipped - 3 lines] > Bill > -- Ferme le Bush I have a scotoma that covers the central field. MD. Very apparent improvement when viewing things with a (say a 7x) telescope. But..... when I get 7x closer physically, the improvement does not seems as much (without the telescope). The scotoma seems to have shifted away from the central field a bit with the scope. Don W. I think you will find it relates to the size of the exit pupil of the telescope. Low vision aids are designed to get as much light into the eye for improved vision. dr grant >I think you will find it relates to the size of the exit pupil of the > telescope. Low vision aids are designed to get as much light into the > eye for improved vision. > > dr grant Dr. Grant, This is the effect I am seeing with a conventional 7x binocular. The exit pupil image enhancement math presently eludes me. The binocular is 7x, I am just looking thru one one side, the scotoma side. Entrance pupil diameter of binocular = 0.75 inch / 19 mm Exit pupil diameter of binocular = 0.43 inch / 10.9 mm Eye pupil diameter = 0.150 inch / 3.8 mm The binocular is a Nikon 7x20 7.1 degrees. So does the math agree with what I am seeing?? Thanks for any inputs. Don W. On 3/11/06 10:22 AM, in article gzEQf.19659$NS6.16991@newssvr30.news.prodigy.com, "Don W" <dwilgus@prodigy.net> wrote: >> I think you will find it relates to the size of the exit pupil of the >> telescope. Low vision aids are designed to get as much light into the [quoted text clipped - 17 lines] > > Don W. If YOUR pupil is indeed 3.8 mm in diameter then the binoculars are indeed setting the exit pupil for the system. With the eye's position at the optimum spot, the exit pupil should have a diameter of 19/7=2.7mm. Most binoculars do not fully utilize the calculated exit pupil. In any event, getting maximum illumination onto the retina is not going to be the limitation when there is a true permanent scotoma. Bill -- Ferme le Bush On 3/10/06 6:19 PM, in article LsqQf.21129$tb3.7014@newssvr24.news.prodigy.net, "Don W" <dwilgus@prodigy.net> wrote: >> There is a good reason for the opposite to be tru and is easy to observe. >> Atmospheric turbulence can really screw up what you see (bad seeing). It [quoted text clipped - 13 lines] > > Don W. I will start by stating I am not a health professional. You made a trick question out of your inquiry by leaving important information out. Are you talking about a permanent scotoma? How bi a field of view does it cover? Do you have a diagnosis. I had some similar problems which later turned out to be ocular migraine. Fortunately the scotoma was temporary. Well, at least until I had a more serious problem that I took to be a recurrence of the migraine. Bill -- Ferme le Bush > I will start by stating I am not a health professional. > [quoted text clipped - 10 lines] > Bill > -- Ferme le Bush Yes, diagnosed as permanent scotoma, as far as I know. Please see above reply for spec. Thanks. Don W. >> I will start by stating I am not a health professional. >> [quoted text clipped - 13 lines] > Yes, diagnosed as permanent scotoma, as far as I know. > Please see above reply for spec. Why is the answer to the question NOT "Because the diameter of the binocular lenses is bigger than the diameter of the lens of the eye"? Did my friend really waste her money on that honking monster lens so she can take pictures in near darkness? Yeah, I'm testy today...  SignatureCheers, Bev ======================================================================= "Lord, grant me the serenity to accept the things that I cannot change, the courage to change the things I can, and the wisdom to hide the bodies of the people who pissed me off." On 3/13/06 8:41 PM, in article PPrRf.253$GM6.113@fe07.lga, "The Real Bev" <bashley101+usenet@gmail.com> wrote: > Why is the answer to the question NOT "Because the diameter of the binocular > lenses is bigger than the diameter of the lens of the eye"? Did my friend > really waste her money on that honking monster lens so she can take pictures > in near darkness? I am not sure what you are getting at because you did not quote the pertinent part. IIRC, the binocular spec quoted by the original poster was 7 X 19, That makes the binocular's exit pupil SMALLER than the eye's pupil. There is an inconsistency in the OP. Bill -- Ferme le Bush > On 3/13/06 8:41 PM, in article PPrRf.253$GM6.113@fe07.lga, "The Real Bev" > <bashley101+usenet@gmail.com> wrote: [quoted text clipped - 8 lines] > X 19, That makes the binocular's exit pupil SMALLER than the eye's pupil. > There is an inconsistency in the OP. I would guess that all binoculars have roughly the same size exit pupil, the main difference being in focal length and [big] lens diameter. Wrong? SignatureCheers, Bev ********************************************** "I've had a Lucas pacemaker for years and have never experienced any prob On 3/13/06 10:54 PM, in article RMtRf.324$uu6.286@fe06.lga, "The Real Bev" <bashley101+usenet@gmail.com> wrote: > I would guess that all binoculars have roughly the same size exit pupil, the > main difference being in focal length and [big] lens diameter. Wrong? Yes. Wrong. Very often the exit pupil of binoculars is smaller than a dark adapted eye. Bird watchers, for example, may want relatively high magnification without the weight of large optics. Naval officers will want to have "night glasses." To estimate exit pupil size, take the diameter of the objective and divide by the magnification. Thus the exit pupil for 7x50 is about 7mm. Note that almost all optic sizes are measured metrically. I do remember control drawings in which metric sizes wer converted to inches. Bill -- Ferme le Bush WTF is Bev on about? I will assume tha tyou are raving about Don's response about exit pupil diameter. I think Don got the wrong figures. He seems to be measuring the lens diameter, not exit pupil. The exit pupil, Don, is an optical point, in this case probably 12-13mm behind the back of the eyepiece. It can be made longer or nearer depending on what the designer wants to achieve. For example, Eschenbach make low vision aids with exit pupils at 18mm to enable spectacle wear while viewing with the telescope. What is an exit pupil? This is the point that the light crosses that is designed to be where your pupil sits when looking through the system. If you are not at the exit pupil, then your field of view will be smaller. The exit pupil if designed correctly and matches the pupils size will transmit the maximum light possible (especially if the front lens is a large diameter) which would produce the relative improvement in brightness. Don, if you are having issues, you would be well advised to see a low vision expert and get some aids to give adequate viewing for the tasks you need. Low vision aids are different from standard telescopes as they are designed to be smaller and lighter in most cases so the can be easily transported and stored in pockets. They are generally designed to be used monocularly to achieve better magnification. Best of luck dr grant Don, > I will assume tha tyou are raving about Don's response about exit pupil > diameter. > > I think Don got the wrong figures. He seems to be measuring the lens > diameter, not exit pupil. Yes, I did measure the lens diameter only not the exit pupil. > The exit pupil, Don, is an optical point, in this case probably 12-13mm > behind the back of the eyepiece. It can be made longer or nearer [quoted text clipped - 16 lines] > easily transported and stored in pockets. They are generally designed > to be used monocularly to achieve better magnification. Excuse the delay. OK as far as the "exit pupil diameter" sitting 12-13 mm behind the eyepiece. The question (puzzlement) is this. If I look at a visual scene my 5mm scotoma occupies so much of my visual field. Computable. But if I put the 7x's to my eye (blocking the good/better eye) the view is much more enhanced. Much more than if I walked 7x closer, it seems. The central field (5 degrees) is much more clearer. There is the angular multiplication of course involved, but the scotoma just looks reduced significantly. The low vision specialist is not considered now since the better eye is better than 20/20. But the reduction of the scotoma on the binocular (ok monocular) view is the question. Originally I had thought there might be some principle of light magnification (more light) involved that would enhance the central view. But I am gathering the answer is no. Thanks all for comments, Don W. |
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