Enter the powers of the first and second cylinder components into the calculator to determine the power of the cross cylinder lens; this calculator can also evaluate any of the variables given the others are known.

## Cross Cylinder Formula

The following formula is used to calculate the power of a cross cylinder lens:

Power = (C1 - C2) / 2

Variables:

• Power is the power of the cross cylinder lens
• C1 is the power of the first cylinder component
• C2 is the power of the second cylinder component

To calculate the power of a cross cylinder lens, subtract the power of the second cylinder component from the power of the first cylinder component. Divide the result by 2 to obtain the power of the cross cylinder lens.

## What is a Cross Cylinder?

A Cross Cylinder is a type of lens used primarily in optometry and ophthalmology for the purpose of refining a patient’s eyeglass prescription. It is a cylindrical lens that is characterized by having one meridian with plus power and the other with an equal amount of minus power, oriented 90 degrees apart. This unique design allows the lens to simultaneously correct for both astigmatism and spherical refractive errors. During an eye examination, the optometrist or ophthalmologist will place the cross cylinder lens in front of the patient’s eye and rotate it to different orientations, asking the patient to compare the clarity of their vision with each change. This process helps the eye care professional to determine the precise degree and axis of any astigmatism present, thereby enabling them to prescribe the most accurate corrective lenses for the patient.

## How to Calculate Cross Cylinder?

The following steps outline how to calculate the Cross Cylinder:

1. First, determine the power of the sphere in the first meridian (S1).
2. Next, determine the power of the sphere in the second meridian (S2).
3. Next, determine the power of the cylinder in the first meridian (C1).
4. Next, determine the power of the cylinder in the second meridian (C2).
5. Finally, calculate the Cross Cylinder using the formula: CC = (S1 – S2) + (C1 – C2).

Example Problem:

Use the following variables as an example problem to test your knowledge:

Power of the sphere in the first meridian (S1) = -2.50 D

Power of the sphere in the second meridian (S2) = -1.75 D

Power of the cylinder in the first meridian (C1) = -1.25 D

Power of the cylinder in the second meridian (C2) = -0.75 D