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% 
