If residual cylinder is less than -0.75 D OR less than 25% of overall spherical refractive error, using sphere is okay.

• A. -0.75 D, 25%
• B. -0.50 D, 15%
• C. -0.25 D, 5%
• D. -1.00 D, 40%

Answer: (A)

When deciding if a spherical lens can be used despite astigmatism, you compare the remaining cylinder (residual cyl) to a commonly accepted cutoff and also consider how large that cylinder is relative to the total spherical error. The idea is to keep residual astigmatism small enough that a spherical lens will still give acceptable vision.

The pair that sits at the acceptable boundary is a residual cylinder of -0.75 D with 25% of the overall spherical refractive error. This choice meets the threshold for both how much cylinder remains and how large it is relative to the spherical error, so using a spherical lens remains a reasonable option.

The other options either show a cylinder power more negative than the cutoff, or a combination that exceeds the allowable proportion of cylinder to spherical error, which would make a spherical lens less suitable.