Which spherical aberration is most likely to affect low to medium powered lenses?

• A. Distortion
• B. Radial Astigmatism
• C. Coma
• D. Longitudinal spherical aberration

Answer: (B)

Radial astigmatism describes how astigmatic error changes with distance from the optical center, so the blur varies across the pupil. In glasses with low to intermediate powers, the overall spherical shaping is mild, so the pure spherical aberration isn’t the dominant image-degrading factor. However, the way rays at different field angles are bent by the lens surfaces creates a disparity between the sagittal and tangential focal lines that grows with radius from center. That field-dependent, radius-sensitive error—radial astigmatism—tends to be the main residual blur in these power ranges, especially with larger pupils.

Distortion and coma are more about how the image is mapped across the field or how off-axis rays are misdirected, and they’re not inherently tied to low-to-medium lens power in the same way. Longitudinal spherical aberration is a spherical aberration that tends to be more problematic with higher powers or thicker lenses where peripheral rays focus far from the paraxial focus. Thus, radial astigmatism is the most likely to affect low to medium powered lenses.