IJCAHPO Certified Ophthalmic Assistant (COA) Practice Exam

To convert the prescription from plus cylinder to minus cylinder form, you need to apply specific rules related to the conversion process. The provided prescription, -2.50 +1.00 X 175, indicates that there is a spherical component of -2.50 diopters and a cylindrical component of +1.00 diopters at an axis of 175 degrees.

In converting to minus cylinder, you need to adjust the spherical component while changing the sign of the cylindrical component. The general formula for this is as follows:

1. Subtract the cylindrical value from the spherical value to find the new spherical value.

2. Change the cylindrical value's sign (from + to -) and adjust the axis by 90 degrees. If the new axis exceeds 180 degrees, you need to subtract 180 degrees to keep it within the common range.

Applying this to the given prescription:

1. From -2.50 (spherical) and +1.00 (cylindrical), you calculate:

-2.50 - 1.00 = -3.50 (this becomes the new spherical power).

2. The cylindrical power changes from +1.00 to -1.00, and the axis changes:

175 +