Coma

Coma (short for comatic aberration) causes point light sources — especially stars — to appear as small comet or bird-wing shapes instead of sharp points. It is worst in the corners of the frame and at wide apertures.

Coma is caused by off-axis light rays hitting the lens at an angle. Rays passing through different zones of the lens converge at slightly different points, smearing the image radially outward from center.

For nightscape and astrophotography, coma is the most important optical flaw to evaluate. A lens with strong coma turns corner stars into distracting smears, ruining widefield Milky Way shots. Coma cannot be corrected in post-processing.

Stopping down reduces coma significantly — most lenses improve dramatically by f/2.8. But nightscape shooters need wide apertures for light gathering, so a lens that controls coma wide open is far more valuable than one that only sharpens stopped down.

In the Wuseria scoring system, coma is rated 0-2 based on corner star rendering at maximum aperture. A score of 2 means minimal coma even wide open — stars stay tight points across the frame. A score of 0 means heavy coma that makes the lens unsuitable for astrophotography without stopping down.