What coma is
Coma, or comatic aberration, is an optical flaw in which points of light away from the center of the frame are drawn as small comets, with a bright head and a fan-shaped tail, instead of round dots. The name comes from the comet-like shape. It shows up most clearly on stars and bright point lights such as street lamps, which is why night photographers notice it.
Points near the center of the image stay mostly round, while the effect grows toward the corners. This happens because light rays entering the lens at an angle do not all focus at the same spot on the sensor.
What causes coma
Coma comes from the lens design. Lenses are built from multiple elements to bring light to focus, and designers balance several aberrations at once. Several things can make coma more visible:
- Wide apertures. Opening the lens lets in more of the outer rays, which are the ones most affected.
- Wide field of view. Wide-angle lenses cover more off-axis area.
- Lens design. Some lenses are better corrected than others, and price is not a perfect guide.
- Sensor size. Larger sensors use more of the image circle, so corner effects are more visible.
Tails that point toward the image center are sometimes called internal coma, while tails pointing away are external coma, depending on the lens design.
Coma versus other aberrations
PAA results ask about lens aberrations in general. Lenses can show several types of imperfection:
- Chromatic aberration causes colored fringes, often purple or green, around high-contrast edges because different colors focus at slightly different points.
- Spherical aberration blurs the whole image slightly because rays through the edge and center of a lens focus at different distances.
- Astigmatism stretches off-axis points in different directions, usually as lines pointing toward or around the center. It can look similar to coma, so it is easy to confuse them.
- Field curvature means the sharp focus surface is curved instead of flat, so corners can be soft when the center is sharp.
- Distortion bends straight lines without necessarily blurring detail.
Telling them apart can be difficult, and a lens often shows more than one at the same time.
What about coma in space?
In astronomy, a comet also has a “coma”, the cloud of gas and dust around its nucleus. This is a separate meaning of the word. Optical coma and cometary coma share a name because of the similar shape, but they are unrelated.
How to spot coma in your photos
- Take a photo of a dark sky with a wide aperture.
- Zoom in to 100 percent on stars in the corners.
- Compare them with stars in the center.
If corner stars have wings or tails and central stars are round, coma is a likely cause. Make sure focus is accurate first, and that the camera is not moving, since focus errors and tracking blur can look similar.
How to reduce coma
Stop down
Closing the aperture by one or two stops usually reduces coma noticeably, since fewer extreme rays pass through. Many photographers try f/2.8 on an f/1.8 lens. The trade-off is that less light reaches the sensor, which may require a higher ISO or a longer exposure. Check the limits for a longer exposure with the 500 rule.
Choose a better-corrected lens
Lenses sold for astrophotography often advertise controlled coma, and independent reviews usually show corner star shapes. Some wide primes handle coma well. Reading reviews and sample images at the aperture you plan to use is the best way to judge. Camera lens basics covers general terms for comparing lenses. A prime lens is not automatically better than a zoom.
Focus carefully
Accurate manual focus on a star near the center is a good start. Some lenses focus best slightly differently at the edges, so check test frames. Focusing issues can make the effect look worse than the lens deserves.
Compose around it
If a particular lens shows strong coma at the edges, keep important stars toward the middle, or crop the corners away.
Correct it in software
Lens correction profiles in editing programs correct distortion and vignetting, but they do not fix coma in general. Some dedicated astrophotography tools offer corner correction, which can reduce the effect on stacked images. Results depend on the tool and the image.
Stack and crop
Taking several frames and stacking them reduces noise, but it does not remove coma. Cropping a bit off the edges is a simple way to lose the worst corners if composition allows.
When coma does not matter
Many viewers do not notice slight coma, especially at normal screen sizes. If your picture is mostly for social media or small prints, a little coma may be acceptable. For large prints or detailed star fields, it is worth testing lenses.
Related guides
Coma is one reason wide-open settings can disappoint, so see Milky Way camera settings and the night photography settings guide for how aperture fits into the whole exposure.