Which of the following best describes radial astigmatism?

• A. A uniform focal shift across all meridians
• B. A difference between tangential and sagittal foci that depends on radial distance
• C. An error corrected by pantoscopic tilt alone
• D. A type of distortion

Answer: (B)

Radial astigmatism is about how the two focal lines in astigmatic optics behave as you move away from the center. In astigmatism there are tangential and sagittal foci, representing how rays in two principal meridians come to focus. If the separation between these two foci stayed the same no matter where you measure from the optical axis, that would be a fixed, uniform astigmatic error. But in radial astigmatism, that separation changes with radial distance from the center—the difference between tangential and sagittal foci depends on how far from the axis you are. This field-dependent behavior is what defines radial astigmatism.

This is why the best description is that the difference between tangential and sagittal foci varies with radial distance. The other options don’t capture this radius-dependent change: a uniform focal shift across all meridians would imply a simple spherical change, pantoscopic tilt alone doesn’t describe a radius-dependent difference, and calling it a distortion is too vague.