Help caculating power in oblique meridians

[Ashlee]

Oaky people heres the scoop I failed my ABO exam in may by 8 points and I am retaking it Sunday Sep. 19th, and I can not figure out how to caculate power in the oblique meridians to save my life. I just do not get it and its embarassing because I should already know this. I know a cylinder exerts 100% of its power at 90 degrees from its axis. 75% @ 60 degrees, 50% @ 45 degrees, 25% @ 30 degrees, and 0% of power on axis. Could someone please walk me through step by step on how to solve a few of these problems. The book im working through gives sample problems and answers but does not walk one throught the steps on how to get the correct answer.

power @ 90 power @ 180
+5.00 +2.50x180 +7.50 +5.00

-2.50 +2.50x090 -2.50 PL

-12.75-4.50x045 -15.00 -15.00

Any and all help would be greatly appreciated.

[lensgrinder]

1.)+5.00 +2.50x180

We know that 100% of the cylinder acts 90 degrees, which you stated.
100% of +2.50 is +2.50 added to the sphere is:
5.00 + 2.50 = +7.50 @ 90 because 90 is 90 degrees away from 180.

What would the power be @ 135 for the same Rx?

135 is 45 degrees away from 180. 45 is 50% of the cylinder
50% of + 2.50 is +1.25 now add this to the sphere
5.00 + 1.25 = +6.25

2.)-2.50 +2.50x090
What is the power at 45?
45 is 45 degrees away from 90.
45 degrees away is 50% of cyl
50% of +2.50 is +1.25 now add +1.25 to the sphere
-2.50 + 1.25 = -1.25 @ 45
What is the power at 30
30 is 60 degrees away from 90.
60 is 75% of cylinder
75% of +2.50 is +1.87. Now add this to your sphere
-2.50 + 1.87 = -0.63 or -0.62

3.)-12.75-4.50x045

Power at 90. 90 is 45 degrees away from 45. 45 is 50% of the cylinder.
50% of -4.50 is -2.25 now add this to your sphere
-12.75 +(-2.25) = -15.00

Hope this helps

[finefocus]

I don't understand your grid, so I'll make a new one.

FOR THE RX -1.00 -2.00 x 180
power at 180 = the sphere plus none of the cyl = -1.00 combined with zero = -1.00
power at 90 = the sphere plus all of the cyl = -1.00 combined with -2.00 = -3.00
power at 45 = the sphere plus half the cyl = -1.00 combined with -1.00 = -2.00

FOR THE RX +1.00 -2.00 x 180
power at 180 = same as above = +1.00
90 = -1.00
45 = plano

FOR THE RX -1.00 + 2.00 x 180
power at 180 =-1.00
90 = +1.00
45 = plano

FOR THE RX +1.00 +2.00 x 180
power at 180 = +1.00
90 = +3.00
45 = +2.00

[Diane]

The change is based on 90 degrees and is 1/90th of the cylinder power for every degree away. The formula can be used to calculate the power of the cylinder in any meridian away from the axis. The formula is the power of the cylinder in any meridian is equal to the original cylinder power multiplied by the sine squared of the angle between the axis and the meridian.
This is a simplified version, and is pretty accurate. Memorize: 0124567889988765421.
Rather than having to work the more difficult formula, simply create the percentage column by adding the numbers in the second column as you go.

0 + 1 = 1 % at 5 degrees away. For 10 degrees away, simply add 2 + 1 and the result is 3 % at 10 degrees away. And so forth. Quick memorization is that 45 is ½ of 90 so 50% of the cylinder power is 45 degrees away. And so forth. For example if the axis given was 30, and you wanted to find the power in the 90th meridian, you would have to find the power 60 degrees away. If you didn’t have the chart and needed to make on, you would begin writing all of the numbers beginning with 0. 0+1+2+4+5+6+7+8+8+9+9+8+8 = 75 % of the cylinder is located 60 degrees away.


Because this chart is in 5 degree increments, it is important not to round off until the final calculation.



I'll upload a chart.

Diane