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Medical Forum / General / Vision / October 2005Question about cylinder axis
Could one of the good eye doctors kindly answer this for me: Let's say I have a cylindrical correction at 45 degrees in the lens over my right eye. Considering a Cartesian coordinate system, and assuming the axis of the cylindrical aspect of my eyeglass lens is indicated by a 45 degree line drawn on the lens, would I see, as I look out through the lens, the line in upper right or upper left quadrant? The question relates to whether the eyeglasses are considered as being viewed from the wearer's side, or the world side. Is the convention the same for the left eye as for the right, or flipped horizontally in the interest of bilateral symmetry? Thanks in advance for any help on this. -- Dicky (Dick Adams) <firstname> dot <lastname> at bigfoot dot com > Let's say I have a cylindrical correction at 45 degrees in the > lens over my right eye. > Considering a Cartesian coordinate system, and assuming the > axis of the cylindrical aspect of my eyeglass lens is indicated > by a 45 degree line drawn on the lens, would I see, as I > look out through the lens, the line in upper right or upper left > quadrant? For both eyes, looking in or out, zero is toward the left ear. By geometry, 0 and 180 are the same meridian so we say 180. 90 is always up, so 45 points up over the left ear. In real eyes, the prescription axis usually shows a mirror-image symmetry - if one eye is 45, the other eye will be in the 135 quadrant. Usually. -MT > In real eyes, the prescription axis usually shows a mirror-image symmetry - > if one eye is 45, the other eye will be in the 135 quadrant. Usually. Interesting. My prescription supports that statement (they're 15 and 155).  SignatureRegards, wK > [ ... ] > For both eyes, looking in or out, zero is toward the left ear. > [quoted text clipped - 4 lines] > In real eyes, the prescription axis usually shows a mirror-image symmetry - > if one eye is 45, the other eye will be in the 135 quadrant. Usually. Thank you. I have recorded that. Now one more question, please: If I orient a grid, like a square of graph paper, so that one set of lines is sharp at some distance d (cm), then move my eye to the distance d' where the other set of lines are sharp, is it true that the amount of my astigmatism is [100/d - 100/d'] ? Or what? Thanks again. -- Dicky Yes. The gap between the first meridian that is clear, and the second meridian 90 degrees away when it is clear, is called "The Interval of Sturm". Your method is accurate if you could hold the axis of the grid exacly on the axis of your astigmatism, which is very hard to do. You can get a very good estimate of your astigmatism by looking at a grid as you described. > If I orient a grid, like a square of graph paper, so that one set of lines > is sharp at some distance d (cm), then move my eye to the distance d' > where the other set of lines are sharp, is it true that the amount of > my astigmatism is > > [100/d - 100/d'] Yes, assuming accommodation remains unchanged via cycloplegia, pseudophakia or some other mechanism... w.stacy,o.d. > If I orient a grid, like a square of graph paper, so that one set of lines > is sharp at some distance d (cm), then move my eye to the distance d' > where the other set of lines are sharp, is it true that the amount of > my astigmatism is > [100/d - 100/d'] I believe so. Notice that you get the same quantity if d' comes first. One way leads to spectacle prescriptions with "plus cylinder" notation, the other leads to "minus cylinder." I use an "astigmatic clock dial." http://www.vision-training.com/Vision%20test/Astigmatism%20test.htm -MT [After a previous post that said:] ----- For both eyes, looking in or out, zero is toward the left ear. By geometry, 0 and 180 are the same meridian so we say 180. 90 is always up, so 45 points up over the left ear. ----- >I use an "astigmatic clock dial." >http://www.vision-training.com/Vision%20test/Astigmatism%20test.htm When I look at that graphic, 0 is toward my right ear, but 90 is still up. Just when I thought I understood this... In your first post, you did mean that zero is toward the left ear of the person wearing the lenses? Or did you mean toward the doctor's left ear as he looks at the wearer? When looking at the "clock dial", how does one relate the numbers to what one sees? Read the number in the direction you can see clearly from the farthest away? Did you know the outer boundary of the lines in that graphic is taller than it is wide (550 H, 518 W), while the open circle in the center is wider than it is tall (85 H, 90 W)? And some of the lines really are much sharper than others (the 180 line is really bad). Is this just someone's sloppy art, or is there some subtle reason for biasing the shapes? Strangely for someone who can't make any of those lines hold still long enough to judge which are sharpest, I'm extremely sensitive to errors in symmetry... Loren > In your first post, you did mean that zero is toward the left ear of > the person wearing the lenses? Yes. > Or did you mean toward the doctor's > left ear as he looks at the wearer? No. > When looking at the "clock dial", how does one relate the numbers to > what one sees? Read the number in the direction you can see clearly > from the farthest away? It won't work unless you have significant astigmatism, and it works differently for different ranges of astigmatism. In the ideal circumstances, people with astigmatism might see the horizontal lines MUCH better than vertical lines, for instance. But bring the target up close, and suddenly the VERTICAL lines are clearer than the horizontal. That's exactly how it works for a 45-year old with a prescription like pl-200x090. When conditions are right, the clockdial effect is dramatic and easy to identify. When our wearer tilts his head 45 degrees, the "bolder" lines shift almost 45 degrees in the same direction. > Did you know the outer boundary of the lines in that graphic is taller > than it is wide (550 H, 518 W), while the open circle in the center is > wider than it is tall (85 H, 90 W)? And some of the lines really are > much sharper than others (the 180 line is really bad). Is this just > someone's sloppy art, or is there some subtle reason for biasing the > shapes? It's sloppy and the rest of the site was junk too. But it loaded quick and it's the first example I came across in a google search. > Strangely for someone who can't make any of those lines hold still > long enough to judge which are sharpest, I'm extremely sensitive to > errors in symmetry... When you don't have much astigmatism it isn't so obvious. -MT |
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