A patient has Ks 43/43.50 at 090, back curve 43D (7.85 mm), and a refraction of -1.75 -0.50 x 180. What contact lens power is required to fully correct the refractive error?

• A. -1.25
• B. -1.50
• C. -1.75
• D. -2.00

Answer: (C)

The main idea is that with gas-permeable lenses, the power you select is driven by the spherical component of the refraction because the lens rests directly on the cornea and there is negligible vertex distance. The cylindrical correction can often be addressed by the tear layer under the lens or, if needed, by a toric GP design, but a spherical GP can still effectively neutralize most of the spherical error.

Here, the refraction shows a spherical correction of −1.75 D and a small cylindrical component of −0.50 D at 180. The corneal curvature is around 43 D with a back curve around 7.85 mm, which is compatible with a spherical GP lens. Because the spherical component is the dominant correction and the astigmatism is minor, selecting a spherical GP lens with the same spherical power as the refraction—−1.75 D—provides full spherical correction. The small −0.50 D cyl is typically managed by the tear lens or by a toric option if needed, but it doesn’t necessitate changing the spherical power from the manifest sphere.

If you used a more positive power like −1.50, you’d under-correct the sphere; with −2.00, you’d over-correct. Therefore, the best match to fully correct the refractive error in this case is −1.75 D.