Which aberration is commonly described as producing a comet-like blur for off-axis point sources?

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

Answer: (A)

Coma is the aberration described as producing a comet-like blur for off-axis point sources. When a point of light sits away from the optical axis, rays entering the eye come from more oblique angles and encounter the curved refracting surfaces in a way that makes central and peripheral rays focalize at different points. This asymmetric spread creates a tail-like extension of the point, resembling a comet. The effect grows with how far off-axis the source is and with larger pupil sizes, so it’s particularly noticeable for off-center objects or when the pupil is wide.

Spherical aberration would blur on-axis points more uniformly because marginal and paraxial rays don’t focus at the same point along the axis, leading to a more symmetric blur. Astigmatism turns a point into a line or oriented blur depending on focus, not a tail. Chromatic aberration causes color fringes around edges due to wavelength differences, not a comet-shaped blur.