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Cylinder power

Optical cross - used to diagram prescriptions
 
 
 
Prescriptions with cylinder axes of 180 or 90 will have all or none of the cylinder power in the  meridian of concern.  Prescriptions with oblique cylinders will only have a percentage of the cylinder power at 180 or 90. 

It would be impractical to memorize the percentage of power in effect at every increment of a cylinder from zero to 90 degrees.  Instead the following chart may be used as a handy reference.

Example #1 -- Using the chart to determine the percentage of oblique cylinder at axis 180.

Rx   +2.00 +5.00 x 30

Depending on the direction moved axis 30 deg is either 30 or 150 degrees from  the 180 meridian of concern.  Refer to column 1 for 30 to 150 line, now refer across to column 2 which gives a percentage to the corresponding line.  In this case the corresponding percentage is 25%.  Multiply the cylinder power by the percentage.
+5.00 X .25 = +1.25
In this case +1.25 diopters of cylinder power is in effect at the 180 line.  Now, algebraically add the effective cylinder to the sphere power for the total which in this case would be

+2.00(sphere) + 1.25 (effective cylinder) = +3.25 (total power at 180)
 
 

Degree from Axis
Percentage of Cylinder Power
0 or 180
0
5 or 175
1%
10 or 170
3%
15 or 165
7%
20 or 160
12%
25 or 155
18%
30 or 150
25%
35 or 145
33%
40 or 140
41%
45 or 135
50%
50 or 130
59%
55 or 125
67%
60 or 120
75%
65 or 115
82%
70 or 110
88%
75 or 105
93%
80 or 100
97%
85 or 95
99%
90 or 90 100%

 

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