Transpose -4.00 +1.25 X 135 into a minus cylinder prescription.

Transpose -4.00 +1.25 X 135 into a minus cylinder prescription.

• A. -2.50 -0.75 X45
• B. -3.00 -2.25 X45
• C. -2.75 -1.25 X45
• D. -2.75 +1.25 X45

Answer: (C)

Converting a plus-cylinder notation to minus-cylinder keeps the same optical power by adjusting the sphere, changing the cylinder sign, and rotating the axis by 90 degrees. For this prescription, the sphere is -4.00 and the cylinder is +1.25 at 135. Add the cylinder to the sphere: -4.00 + 1.25 = -2.75. Change the cylinder to negative: -1.25. Rotate the axis by 90 degrees: 135 + 90 = 225, which wraps to 45. So the minus-cylinder prescription is -2.75 -1.25 X45.

Converting a plus-cylinder notation to minus-cylinder keeps the same optical power by adjusting the sphere, changing the cylinder sign, and rotating the axis by 90 degrees. For this prescription, the sphere is -4.00 and the cylinder is +1.25 at 135. Add the cylinder to the sphere: -4.00 + 1.25 = -2.75. Change the cylinder to negative: -1.25. Rotate the axis by 90 degrees: 135 + 90 = 225, which wraps to 45. So the minus-cylinder prescription is -2.75 -1.25 X45.