answers to lens cross lesson

Answers to optical cross problems

PUTTING THE PRESCRIPTION ON THE OPTICAL CROSS

Combine the sphere reading and the axis of the prescription.
Add together the sphere and the cylinder amount.
If the axis is less than or equal to 90, add 90 to it. If the axis is greater than 90, subtract 90 from it.
Combine the power in step 2 with the axis in step 3.

The sphere and the axis go together. So, on one leg of your diagram you should have +5.00 and 135.
Sphere plus cylinder is +5.00 + (+1.00) = +6.00.
135 is greater than 90, so we subtract 90 from it. 135 - 90 = 45.
Combine +6.00 and 45 on the other leg of the diagram.

+5.00 and 135

+6.00 and 45

The sphere and the axis go together. So, on one leg of your diagram you should have -1.25 and 45.
Sphere plus cylinder is -1.25 + (-3.25) = -4.50.
045 is less than 90, so we add 90 to it. 45 + 90 = 135.
Combine -4.50 and 135 on the other leg of the diagram.

-1.25 and 45

-4.50 and 135

The sphere and the axis go together. So, on one leg of your diagram you should have +3.50 and 90.
Sphere plus cylinder is +3.50 + (-3.50) = 0.00.
090 is equal to 90, so we add 90 to it. 90 + 90 = 180.
Combine pl and 180 on the other leg of the diagram.

+3.50 and 90

pl and 180

The sphere and the axis go together. So, on one leg of your diagram you should have pl and 110.
Sphere plus cylinder is 0.00 + (-1.25) = -1.25.
110 is greater than 90, so we subtract 90 from it. 110 - 90 = 20.
Combine -1.25 and 20 on the other leg of the diagram.

pl and 110

-1.25 and 20

The sphere and the axis go together. So, on one leg of your diagram you should have +0.50 and 115.
Sphere plus cylinder is +0.50 + (-1.75) = -1.25.
115 is greater than 90, so we subtract 90 from it. 115 - 90 = 25.
Combine -1.25 and 25 on the other leg of the diagram.

+0.50 and 115

-1.25 and 25

TAKING THE PRESCRIPTION OFF THE OPTICAL CROSS

If you are doing minus cylinder form, choose the most plus or least minus -- the highest one on the scale at the bottom of page 43.
If you are doing plus cylinder form, choose the least plus or most minus -- the lowest one on the scale at the bottom of page 43

Write the prescription in minus cylinder form:

6.

Choose the power that is highest on the scale on page 43: +3.75.
+3.75 x180
Subtract +3.75 from +2.75: +2.75 - (+3.75) = -1.00. This is the amount of the cylinder. On the number line on page 43 you have to go 1.00 in the minus direction to get from +3.75 to +2.75.
+3.75 -1.00 x180

Choose the power that is highest on the scale on page 43: -1.50.
-1.50 x45
Subtract -1.50 from -3.75: -3.75 - (-1.50) = -3.75 + 1.50 = -2.25. This is the amount of the cylinder. On the number line on page 43 you have to go 2.25 in the minus direction to get from -1.50 to -3.75.
-1.50 -2.25 x045

Choose the power that is highest on the scale on page 43: +0.25.
+0.25 x025
Subtract +0.25 from pl (or 0.00): 0.00 - (+0.25) = -0.25. This is the amount of the cylinder. On the number line on page 43 you have to go 0.25 in the minus direction to get from +0.25 to 0.00.
+0.25 -0.25 x025

Choose the power that is lowest on the scale on page 43: -0.25.
-0.25 x045
Subtract -0.25 from +0.25: +0.25 - (-0.25) = +0.25 +0.25 = +0.50. This is the amount of the cylinder. On the number line on page 43 you have to go 0.50 in the plus direction to get from -0.25 to +0.25.
-0.25 +0.50 x045

10.

Choose the power that is lowest on the scale on page 43: pl.
pl x062
Subtract -0.00 from +3.00: +3.00 - (0.00) = +3.00. This is the amount of the cylinder. On the number line on page 43 you have to go 3.00 in the plus direction to get from 0.00 to +3.00.
pl +3.00 x062

11.

Choose the power that is lowest on the scale on page 43: -1.50.
-1.50 x180
Subtract -1.50 from -0.25: -0.25 - (-1.50) = +1.25. This is the amount of the cylinder. On the number line on page 43 you have to go 1.25 in the plus direction to get from -1.50 to -0.25.
-1.50 +1.25 x180

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