Which type of aberration is caused by varying magnification based on the height of incident rays above the axis and only occurs for off-axis point sources?

• A. Astigmatism
• B. Coma
• C. Spherical aberration
• D. Chromatic aberration

Answer: (B)

Coma is the field-dependent blur that appears when magnification varies with how far a ray is from the optical axis. An off-axis point source doesn’t form a sharp image because rays farther from the axis are magnified differently, producing a comet-like tail that points away from the axis. This effect only shows up for off-axis objects, since an on-axis point can still image as a point if other aberrations are small.

Astigmatism involves different focal lengths in two meridians, so off-axis points can blur into lines rather than points, but it’s about the lens’s geometric symmetry in handling different directions, not about magnification changing with ray height. Spherical aberration arises when rays at different distances from the axis focus at different depths, leading to blur for on-axis objects rather than the characteristic comet shape from field-dependent magnification. Chromatic aberration comes from dispersion: different wavelengths focus at different points, causing color fringes rather than the off-axis comet-like image.

So the described effect directly matches coma.