Residual astigmatism is the amount of astigmatism that is not corrected by a GP contact lens.

• A. Refractive cylinder minus Corneal cylinder
• B. Corneal cylinder minus Refractive cylinder
• C. Refractive sphere minus Corneal sphere
• D. Total astigmatism minus sphero-cylindrical correction

Answer: (A)

Residual astigmatism is the leftover astigmatic error after a GP lens has neutralized the corneal part. A rigid gas-permeable lens can correct the corneal cylinder, so what remains comes from the refractive cylinder measured in the refraction. Therefore the amount of residual astigmatism is the difference between the refractive cylinder (total astigmatism seen on refraction) and the corneal cylinder (the corneal contribution that the GP lens neutralizes). If the axes align, you simply subtract the corneal cylinder from the refractive cylinder; if not, a vector difference would be needed, but the concept remains the same.

This is why the statement describing residual astigmatism as refractive cylinder minus corneal cylinder best captures the idea. The other ways of framing it—such as subtracting spheres or using total astigmatism minus a sphero-cylindrical correction—don’t reflect how residual astigmatism is defined in relation to the corneal contribution and the GP’s corrective effect.