What is coma in photography? Have you ever seen a star or highlight turn into a tiny comet near the edge of your frame?

Coma is an optical flaw that makes point lights look like little comets instead of round dots. You usually see it at the corners, with wide apertures, and in star or bokeh shots.

This article explains what causes coma, how to spot it in your images, and practical ways to fix or reduce it. You will find simple tests, camera and telescope fixes, and post-processing tips with clear example images.

Follow the short checklist and hands-on tests we give so you can check your own gear today. By the end you’ll know when to stop down, when to correct alignment, and when to crop or repair the image in software.

What is coma in photography?

Coma, short for comatic aberration, is an off-axis optical error that turns tiny points of light into little comets. Instead of a clean dot, you get a teardrop with a tail because the light rays do not converge symmetrically.

Picture a star near the corner of your frame that looks stretched, with the tail pointing away from the center. That lopsided blur is an asymmetric point-spread function, which is the optical way of saying the lens draws a point as a skewed shape.

You will see coma most in the edges and corners of the image, especially at wide apertures. It shows up clearly in astrophotography, but also in background lights and out-of-focus highlights.

If you have ever asked yourself what is coma in photography, think of it as a field-dependent smearing that grows with distance from the center. In the center, points often remain clean and round while the corners grow messy.

Bright city lights, fairy lights, and reflections on water are common daytime or indoor clues. Nightscape shooters notice it right away as stars turn into little comets at the sides of the frame.

An ideal illustration for this section would be a side-by-side of a round center star and a corner star with a clear tail. A short caption can note the radial direction of the tail from the frame center.

What causes coma?

Coma happens because off-axis rays pass through different parts of the lens and bend by different amounts. The lens cannot bring those rays to one clean point, so the image of a point spreads and skews.

With spherical surfaces, this asymmetry gets worse as you move away from the center of the image. Aspheric elements are used to tame it by reshaping how the lens bends light at those off-axis angles.

Large apertures make coma more visible because the lens uses more of its glass area. The farther out in the optical path the rays travel, the more uneven the focus can become.

Wide fields of view also push the limits, since corner rays enter at steeper angles. Fast wide-angle lenses, especially older designs, will show strong coma if not corrected well.

Design matters a lot, but so does build precision. Decentering or tilt of elements, even slight, can amplify coma or make it stronger on one side of the frame.

Sensor and mount alignment can also play a role. A tilted adapter, a loose focusing helicoid, or a misaligned sensor stack can mimic the same skewed star shapes.

Do not confuse coma with spherical aberration, which affects on-axis rays and makes the center soft rather than skewed. Field curvature, on the other hand, shifts the plane of focus so corners look soft unless you refocus, but it does not create a comet tail.

There can also be a mild chromatic component if colors focus slightly differently off-axis. In that case the tail may show a faint color fringing, which is a mix of comatic and chromatic effects.

If you want a deeper dive into the optics background, look up coma in optics. A simple ray sketch with off-axis marginal rays landing at shifted positions is a helpful mental picture.

Coma appearance in images and how to detect it

Coma looks like a star or highlight with a wedge or tail pointing away from the center. The farther out you go, the longer and messier that tail becomes.

Another tell is in bokeh: bright background circles stretch into teardrops near the edges. The direction is usually radial from the frame center, which is a quick ID trick.

For general shooting, zoom to 100% on corner highlights or specular reflections and compare them to the center. A live view magnify test at night, pointed at a streetlight or bright star, will show it fast.

For astrophotography, try a star-field test at several apertures and positions in the frame. Move the same star from center to mid-frame to corner, and compare shapes at each step.

You can also make simple notes on shape, tail length, and direction to track performance. Some astrophotographers measure FWHM, or full width at half maximum, and watch how it stretches asymmetrically in the corners.

Repeat your test over a few frames to rule out tracking or stacking errors. If the tails are always radial and always in the same corners, you are seeing lens behavior, not a drift issue.

If you want a field guide on how to set up repeatable tests, learn how to test lenses for coma. Include a grid of star crops across the frame and a before and after stopped-down example for clarity.

How to avoid and correct coma

The simplest fix is to stop down a bit. A lens at f/1.4 might look much cleaner by f/2 or f/2.8, because you are using less of the outer glass where coma is stronger.

There is a trade-off when you stop down, though. Diffraction will nudge fine details softer at very small apertures, and you will also need longer exposures or higher ISO.

Better optics help a lot. Look for lenses known for strong off-axis correction and check star-field samples, MTF charts, and PSF crops in trusted reviews.

If you already own the lens, do a structured test and see what aperture yields an acceptable corner shape. Many fast primes become excellent by stopping down one or two stops.

Mechanical alignment is just as important as optical design. Check for tilted adapters, loose mount screws, or a decentered element, since tilt can create coma-like streaks on one side.

Astrophotographers have a few more tools for severe cases. Coma correctors, such as Wynne, Baader MPCC, or Paracorr-style units, are designed to neutralize coma in fast telescopes and can transform star shapes.

Some systems add field flatteners or use aplanatic designs that correct coma and spherical aberration together. Active collimation and adaptive optics exist too, though they are advanced and rare for normal camera lens work.

In post, you can try PSF-based deconvolution in tools like PixInsight or StarTools. This may tighten mild tails, but heavy coma will leave artifacts, so be conservative and compare before and after zoomed crops.

When the corners are still messy, a clean crop or a slight reframe is a practical answer. Many nightscapes accept a small crop to protect the central subject and keep stars pleasing where it matters.

For a broader primer on mitigation steps, see how to reduce coma in lens and shooting practice. Use tests to decide if stopping down, using a corrector, or upgrading optics is the most sensible path.

Practical tips, testing checklist and recommended assets to include in the article

Pick a simple star field, focus carefully at the center, and shoot a short exposure wide open. Then repeat at one or two smaller apertures so you can compare shapes.

Move the same bright star or light to mid-frame and then to the corner, and shoot again. Inspect 100% crops and write down where tails start to appear and which direction they point.

If coma looks strong, stop down, check for mount or adapter tilt, and retest. If the pattern stays, consider a corrector for astro or a different lens for critical work.

Use high ISO and short exposures during tests so star shapes are not blurred by motion. Capture a before and after stopped-down pair, plus a decentering example if you can reproduce one.

Include clear 100% center, mid, and corner crops, plus a simple ray-trace sketch labeled “source of coma.” Care most when shooting nightscapes, astro, or wide-angle scenes with point lights; worry less for portraits or soft-lit interiors.

What People Ask Most

Final Thoughts on Coma in Photography

If you remember one number from the tests we outlined, let it be 270 — a simple frame offset many reviewers use to make corner stars reveal their tails. You came here wondering what is coma in photography, and now you’ve got a clear definition, the optical intuition behind off‑axis tails, and practical ways to spot them so your point‑sources stay honest.

The core benefit is confidence: armed with quick star tests and 100% crops, astrophotographers and nightscape shooters will know whether to stop down, adjust alignment, or invest in better optics. But be realistic — stopping down helps a lot but brings diffraction and longer exposures, and software fixes can’t fully restore heavily comatic corners.

We walked through causes, identification methods, and practical fixes so you can run the simple checklist and see results in your own files. Keep experimenting with tests and crops — you’ll refine your kit and eye, and your night shots will show it.