What is coma in astrophotography?

Ever tried to shoot a picture of the night sky and ended up with stars that look just a little off? Does it look like they are “stretched” or have “tails”? That’s coma, also known as a comatic aberration.

In this article, we’ll be running through anything and everything you’ll ever need to know about coma as it pertains to astrophotography. Ready to become a star-shootin’ pro? Let’s dive into it.

A visual representation of coma in astrophotography.
Notice the “stretched” comet-like stars.

What is coma?

Comatic vs Chromatic Aberrations

Before we get too far, let me make one thing clear: comatic aberrations do not equal chromatic aberrations.

Chromatic aberrations are the result of lens dispersion, leading to colored (generally purple or blue) lines along the high-contrast parts of an image. Typically, chromatic aberrations are easily fixed in post-processing with a single click.

Comatic, on the other hand, is permanent. Unfortunately, you can’t fix coma in Photoshop or Lightroom.

Definition of Coma

Alright so, what exactly is coma? Without diving too deep into the science behind it, the general concept is that certain points of light (generally near the edge of the frame) hit the spherical elements of a lens “improperly”.

This is a common problem that is experienced not just by camera lenses, but by all sorts of optical equipment. Binoculars, low-end telescopes, etc.

This phenomenon can be easily observed by shooting a (properly exposed) image of the night sky and then zooming in on the extreme corners of your image. You’ll notice comet-shaped stars with “tails”, as we mentioned earlier.

Man looking up at the stars.

How to avoid coma?

Now that you know what coma is, how do you prevent it?

Stopping Down

The most simple way to reduce coma in astrophotography is to stop down.

Oftentimes, lenses suffer the most aberrations (of any kind, whether comatic or chromatic) when wide open, thus dropping down just a few stops (in most cases) is able to remedy the vast majority of optical imperfections.

However, as anyone who has ever photographed the night sky knows, every single stop of light is valuable. Oftentimes, stopping down isn’t exactly the most appealing option.

Finding an Astro-Friendly Lens

That’s where finding a new lens comes in. I’m a firm believer that “gear doesn’t matter”, but only to a point.

It’s a simple fact that certain lenses, often those built specifically with astrophotography in mind, have vastly better performance when it comes to controlling comatic aberrations.

Speaking of which…

Sony Lenses with Low Coma

Sometimes you just need to upgrade to a better lens. The simple truth of the matter is that some lenses just suck when it comes to astro. It may take a bit of digging across the internet, but many different photographers have reviewed the astrophotography performance of their wide-angle lenses.

Below, you can find links to the absolute best astro lenses for both a6xxx series cameras and a7x bodies. Thanks for reading, now get out there and shoot some stars!

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