Which type of spherical aberration is based on the principle that marginal rays focus to a different location compared to paraxial rays and occurs for both on- and off-axis point sources?

Which type of spherical aberration is based on the principle that marginal rays focus to a different location compared to paraxial rays and occurs for both on- and off-axis point sources?

• A. Longitudinal
• B. Radial
• C. Coma
• D. Curvature of field

Answer: (A)

Longitudinal spherical aberration happens when rays farther from the optical axis (marginal rays) come to focus at a different point along the optical axis than rays close to the axis (paraxial rays). This means a point object isn’t brought to a single sharp plane; the image blur lies along the axis because the focal positions differ by ray height. The effect arises from the way a spherical surface refracts light: its power isn’t the same for all ray heights, so marginal rays and paraxial rays don’t share the same axial focus. This type of aberration affects imaging for both on-axis and off-axis points since the axial misfocus depends on ray height rather than field position. In contrast, transverse (lateral) spherical aberration changes where rays land perpendicular to the axis and mainly shows up off the axis, while curvature of field and coma relate to other ways images get blurred across the field or look comet-like for off-axis points.

Longitudinal spherical aberration happens when rays farther from the optical axis (marginal rays) come to focus at a different point along the optical axis than rays close to the axis (paraxial rays). This means a point object isn’t brought to a single sharp plane; the image blur lies along the axis because the focal positions differ by ray height. The effect arises from the way a spherical surface refracts light: its power isn’t the same for all ray heights, so marginal rays and paraxial rays don’t share the same axial focus. This type of aberration affects imaging for both on-axis and off-axis points since the axial misfocus depends on ray height rather than field position. In contrast, transverse (lateral) spherical aberration changes where rays land perpendicular to the axis and mainly shows up off the axis, while curvature of field and coma relate to other ways images get blurred across the field or look comet-like for off-axis points.