Originally Posted by
Chad Sobodash
I'd use the formula. It's unlikely they'd ask me that, since it doesn't follow neatly into the trigonometric absolute set of 25%, 50%, 75%, and will yield a value that is potentially not in quarter or eighth diopters. However, if they did, let's look at the consequences of using the method you showed on a high prescription:
-12.75 -4.00 x 025
-12.00 -4.25 x 090
Find the power at 90.
0.65 * -4.00 = -2.6
-2.6 + (-12.75) = -15.35
Whereas the actual value derived from (sin(65))^2 is 82%:
0.82 * -4.00 = -3.28
-3.28 + (-12.75) = -16.03
That's over half a diopter difference.
I believe that bad habits die hard, and it's better to just learn the formula and be able to apply it quickly. Maybe I'm mathematically gifted, but I find it easier to remember it this way, and more sensible. I don't notice a difference in speed doing it this way versus your way.
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