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Medical Forum / General / Vision / March 2006Where do they put the cylinder correction?
In this prescription for farsightedness: Sph = +1.75, Cyl = +1.25, Axis = 174 How is the cylinder correction ground into the lens? I.e., is it in the shape of an X at the proper axis? Or is it ground all over the lens at the proper axis? And which way is that correction ground as viewed from the *front* of the lens rather than from the side the eye is on? I.e., does it slant uphill from lower left to upper right, or does is slant uphill from lower right to upper left? > In this prescription for farsightedness: > [quoted text clipped - 3 lines] > I.e., is it in the shape of an X at the proper axis? > Or is it ground all over the lens at the proper axis? It's usually the entire back surface. Instead of spherical, it's a little "football shaped." To understand astigmatism, try this experiment. If you're wearing bifocals, make sure to use the distance portion only. Fold up your glasses, then hold them in place, then close one eye and rotate the frame vertical (not possible without folding the arms.) When you're looking through the lens 90 degrees out of whack, everything will be blurry. That's astigmatism. Rotating your lens 90 deg has the effect of doubling your astigmatism. Rotating your lens _another_ 90 degrees (upside-down) clears things up again. In real lenses, each surface, front and back, have its own power. Glasses are usually made with the front surface convex (+ power) and the back surface concave (- power). The two surfaces add together to give the actual prescription. In your case, the front surface is perhaps +500 and the back surface is roughly -200 along the vertical meridian, and -325 along the horizontal. > And which way is that correction ground as viewed from the *front* of > the lens rather than from the side the eye is on? Pretty much like a football standing on end, rather than on its side or oblique. > I.e., does it slant uphill from lower left to upper right, or does is > slant uphill from lower right to upper left? In the case of your prescription, things should be magnified more in the vertical direction, and less horizontally. A square should lean slightly (left?). That's assuming the center of the lens is at the center of your pupil, and plus lenses inevitably produce "pincushion" distortion that varies everywhere. -MT, OD Now I'm all mixed up :) I thought 174 degrees was almost horizontal, but you're saying it's almost vertical? Which way is straight up and down? I thought that was 90 degrees, and that horizontal was 180 degrees. Are you saying that the pointy ends of the football shaped part ground into it are near the top and bottom of the lens? >> In this prescription for farsightedness: >> [quoted text clipped - 40 lines] > >-MT, OD > Now I'm all mixed up :) I thought 174 degrees was almost horizontal, > but you're saying it's almost vertical? 174 is near 180 and 180 is the horizontal diameter (meridian). The power along the 174 meridian is +175. The power along the vertical (084) meridian is +300. The prescription reads "sphere +175, cylinder +125, axis 174" "Axis 174" means the extra +125 is NOT in the 174 meridian, but 90 degrees away. "AT 174" is the opposite of "AXIS 174." The back surface would fit against a football standing on its end, 6 degrees off of vertical. -MT |
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