Coma is an optical aberration where off-axis point sources appear distorted.
Technical Details
Coma primarily occurs in lenses with wide apertures (f/1.4 to f/2.8) and in the image corners, where the distance to the optical axis is greatest. The strength of the aberration is measured by the coma coefficient (in wavelengths) – values below 0.1λ are considered acceptable. Sagittal and meridional coma differ in their orientation: sagittal coma points radially outward, meridional tangentially to the image circle. Modern lenses reduce coma through aspherical lens elements and special glass types with anomalous partial dispersion.
History & Development
Ernst Abbe first scientifically described coma aberration in 1873 and developed the sine condition for its correction. In 1896, Paul Rudolph designed the first largely coma-corrected lens, the Zeiss Planar. In the 1950s, new glass types from Schott and Ohara enabled improved corrections. Since the 1980s, computer-aided lens calculations and aspherical elements have significantly reduced coma – modern cine lenses like the Zeiss Master Prime series achieve near coma-free imaging even at wide-open apertures.
Practical Use in Film
Coma particularly affects night shots and low-light scenes, where wide apertures are necessary. Roger Deakins deliberately utilized slight coma effects in "Blade Runner 2049" for atmospheric light sources. During Steadicam movements, coma is amplified by the continuous motion – here, lenses like the Angenieux Optimo 24-290mm compensate through special coma correction. In digital post-production, coma can be partially corrected by lens correction tools in DaVinci Resolve or Nuke, but at the cost of image sharpness.
Comparison & Alternatives
Coma differs from spherical aberration due to its asymmetric shape and its occurrence only at off-axis points. While astigmatism distorts point-like light sources into lines, coma creates teardrop-shaped distortions. Modern Master Anamorphic lenses exhibit significantly less coma than vintage anamorphics from the 1960s. Alternative corrections are offered by field flattener systems or by using smaller image sensors with the same field of view, thereby utilizing only the low-coma central lens area.