If keratometry readings are 42.50 @ 090 and 41.75 @ 180, the proper prescription to correct this refractive error would be:

• A. +0.75 @ 090
• B. Plano @ 180
• C. -0.75 @ 090
• D. +0.75 @ 180

Answer: (D)

The cornea shows mild astigmatism because the two principal meridians have different powers: 42.50 D at 090 and 41.75 D at 180. The steeper meridian is 090, and the flatter one is 180. To correct this astigmatism in a spectacle prescription, the cylinder is placed so that its axis lies along the flatter meridian, and the cylinder power is applied to the steeper meridian to neutralize the difference.

Here, the flatter meridian is 180, so the cylinder axis should be 180. Using a plus cylinder of 0.75 diopters adds power to the 90 meridian (the one perpendicular to the axis), raising it from 42.50 to 42.50, while the 180 meridian also becomes 42.50. This neutralizes the astigmatism.

Therefore, the proper prescription is +0.75 at 180.