Residual astigmatism equals refractive cylinder minus corneal cylinder. If refractive cylinder is -2.00D at 180 and corneal cylinder is -2.00D at 180, the residual astigmatism is:

• A. -4.00D
• B. 0D
• C. -2.00D
• D. +2.00D

Answer: (B)

Residual astigmatism is the difference between the refractive cylinder and the corneal cylinder, taking into account both magnitude and axis. If both cylinders are the same magnitude and share the same axis, they cancel each other out.

Here, the refractive cylinder is -2.00 D at 180 and the corneal cylinder is -2.00 D at 180. Subtracting gives -2.00 minus (-2.00) = 0.00 D. So there is no residual astigmatism. If the axis or magnitude differed, you’d combine them vectorially to find the remaining astigmatism, but with identical power and axis, it’s zero.