Aberration occurs when a lens or mirror fails to focus all incident light into a perfect, sharp image. This defect causes blurring, distortion, or color fringing at the edges of a beam. Optical designers use multiple lens elements to correct these deviations and achieve a crisp focal point.
The fixture has too much chromatic aberration at the edges of the beam, leaving a visible blue halo on the wall.
In stage lighting, a little spherical aberration is sometimes desirable to soften the hard edge of a spot.
Related terms
Chromatic aberration
Color fringing caused by a lens failing to focus all wavelengths at the same point.
Coma
An optical aberration where off-axis light rays focus into a comet-like blur rather than a sharp point.
Astigmatism
An optical defect where light rays on different planes focus at different distances.
Distortion
An optical aberration where magnification varies across the field of view.