Against what rule?
Anyone ever heard of some fangled thing called "against the rule astigmatism?" A woman came into our shop recently, and took off her rimless specs. She shows them to me and says " these are thicker here than they are here because of "AGAINST THE RULE ASTIGMATISM." She looked me up and down, then says "well, you certainly wouln't know what that is. My optician does." She leaves, with me standing there pondering the future of the human race. I went to my auto mechanic and says, "this is a spark plug. SPARK PLUG. but you wouln't know what that is." sigh.
This is from Google: In with-the-rule astigmatism, the eye has more refractive power along the vertical axis and the patient has difficulty resolving targets with horizontal lines (e.g., letters such as E or F). A patient with against-the-rule astigmatism has the opposite problem; they have difficulty focusing vertically oriented targets. I would have said, "Yes, clearly I am too stupid to understand a simple optical concept. They did not teach that when I graduated from college." I had to google the answer to this one myself to get a concise definition.
I searched that Optical Google site and found this posted by LensGrinder on Optiboard a while back:
A rule of thumb is;
With the rule-minus cylinder, axis wihtin 30 degrees of the 180th meridian, Or plus cylinder, axis within 30degrees of the 90th meridian.
Against the rule-minus cylinder, axis within 30 degrees of the 90th meridian, or plus cylinder axis within 30 degrees of the 180th meridian.
Oblique-cylinder axis from 31-59, or 121-149
Read the thread here: http://www.optiboard.com/forums/showthread.php?t=14992
so does that mean 90 is with the rule and 180 is against the rule? and what the F difference does it make the lens will be thinner on the axis and thickest 90 away from that.
thanks eyefitwell posted the answer while I was posting the question
Last edited by k12311997; 12-29-2006 at 04:57 PM.
It must have been a full moon in your area.....either that or she's escaped from the nut farm.;)
I didn't realize people had so many questions about the cylinder orientation. If the rx axis is 90, the astigmatism is against the rule, IF the rx is in minus cyl form. As indicated already, in with the rule, the greatest refractive power is vertical, or, at the 90 meridian. Confusion can result from mixing up the actual astigmatism from the CORRECTING cylinder. More confusion results when one considers that a correcting cylinder has NO power at its stated axis. To keep it clear, imagine a can of soup superimposed over someone's eye. If it is horizontal, the greatest curvature is up and down, while the horizontal has no curve at all. This is WTR astigmatism. If someone's cornea were so shaped, you would need a CORRECTING cylinder opposite to that, or with the greatest curve horizontal. Since the correcting cyl has no power on its stated axis, the axis would be stated as the meridian which has the least power, which is 180. Better still, just remeber, if the rx shows axis 180 or therabouts, and the rx is in minus cyl, it is with the rule, or WTR.
Simply put the WTR is flatest horizontal ATR is flatest vertical. We are almost all born ATR and by I want to say 5-6 years old most of us will end up WTR, the theory is that the upper lid is tight causeing the vertical meridian to steepen, then as we age (over 65 or so) the opposite occurs the lids loosen and the shift goes back to ATR (not in all cases so don't shoot me). Also look up Javal's Rule for a interesting tidbid on the curvature of the cornea. It basically states that a spherical refractive error will have K's of -0.50@090 .
The simple theory is cylinder at least in young people is formed by lid pressure. Lid pressure is in the 90 degree meridian so most people have the shortest corneal radii in the 90 degree meridian. Some of us don't conform to this and have the shortest meridian in the 180 degree meridian. That's up and down for the 90 and side to side for the 180.
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