A patient's keratometry readings are 43.00D @180 and 45.00D @090 with subjective refraction plano -2.00 x 180. What is their Residual astigmatism?

A patient's keratometry readings are 43.00D @180 and 45.00D @090 with subjective refraction plano -2.00 x 180. What is their Residual astigmatism?

• A. 0D
• B. 1D
• C. 2D
• D. -1D

Answer: (A)

Keratometry showing 43.00 D at 180 and 45.00 D at 090 means the cornea has 2 diopters of astigmatism, with the steeper (more power) meridian along 90 degrees. In minus-cylinder notation, that corneal astigmatism is equivalent to -2.00 D with axis 180 (cylinder power applied perpendicular to the axis). The subjective refraction given is plano -2.00 x 180, which provides exactly that same cylindrical correction along the 90-degree meridian. Since the refractive cylinder cancels the corneal cylinder, there is no remaining astigmatic error. Residual astigmatism is 0 D.

Keratometry showing 43.00 D at 180 and 45.00 D at 090 means the cornea has 2 diopters of astigmatism, with the steeper (more power) meridian along 90 degrees. In minus-cylinder notation, that corneal astigmatism is equivalent to -2.00 D with axis 180 (cylinder power applied perpendicular to the axis). The subjective refraction given is plano -2.00 x 180, which provides exactly that same cylindrical correction along the 90-degree meridian. Since the refractive cylinder cancels the corneal cylinder, there is no remaining astigmatic error. Residual astigmatism is 0 D.