Which aberration occurs for off-axis point sources specifically?

• A. Coma
• B. Longitudinal spherical aberration
• C. Astigmatism
• D. Chromatic aberration

Answer: (A)

Off-axis point sources create a comet-like blur, not a round point. This happens because rays coming from a point off the optical axis strike the lens at different heights across the aperture and are bent in a way that isn’t symmetric about the image center. The result is an elongated blur with a tail that points radially away from the optical axis. The effect, called coma, becomes more pronounced as you move farther from the center of the field or use a faster (lower f-number) optic. By contrast, longitudinal spherical aberration is an axial issue that blurs on-axis points symmetrically along the optical axis, astigmatism produces directional blur into line shapes due to different focusing in perpendicular planes, and chromatic aberration is a color-dependent focal shift that affects all field positions similarly rather than creating a tail-like shape for off-axis points.