Testing Lenses For Coma

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anonymous, Lens-coma, CC BY-SA 3.0
A ray-traced drawing showing how off-axis light is misfocused in a lens with coma. Source: anonymous, Lens-coma, CC BY-SA 3.0

As a nightscape and Milky Way Photographer, one of the top criteria I use when selecting lenses is to look at the shape of stars when shooting at wider apertures. Their shape is affected by several lens aberrations. And they are most apparent in the corner of the image. Coma is one of those aberrations. And you don’t need any fancy gear to be testing lenses for coma.

Testing Lenses For Coma - Photo Credit: No machine-readable author provided. MetaNest assumed (based on copyright claims)., ComaAberrationSample, CC BY-SA 3.0
A close-up of a lens coma aberration.
Source: Wikimedia

All lenses are affected by coma, or technically comatic aberration, to some degree. Faster lenses with a wide-angle of view, like those typically used by night photographers, are often affected by coma. Coma can even vary with the color of light, which then makes it a form of chromatic aberration.

Coma only affects light rays that pass through a lens at an angle. You can combat it by stopping down the aperture, but you’ll want to do some of your own testing lenses For coma to find out just how much is needed.

Look to the Corners

When stars fall in the corner of the image frame, coma deforms their shape, causing them to have what looks like an out-of-focus “comet tail,” even when the lens is focused correctly. These comet tails typically radiate in a line away from the center of the image, creating stars that are not pinpoint shaped. Excessive coma gives results that are less than desirable! Hence the desire to avoid lenses that suffer significantly from it.

Coma should not be confused with sagittal or tangential astigmatism, another type of lens aberration, which can create shapes that look like bat or angel wings extending from the stars. But commonly, some amount of coma and sagittal or tangential astigmatism is present in a fast, wide lens. This testing will show how to minimize them in your photographs.

Cutting Coma

Fortunately, there’s a way to reduce coma. Stopping down the aperture to a larger f/number blocks the light rays from entering the lateral edge of the lens and reduces coma. And there’s the catch, for nightscaper and Milky Way photography, we try to avoid stopping down excessively.

anonymous, Lens-coma, CC BY-SA 3.0
A ray-traced drawing showing how off-axis light is misfocused in a lens with coma.
Source: anonymous, Lens-coma, CC BY-SA 3.0

So, it is helpful to know the least amount of stopping down needed to remove coma. It’s actually easy. You take photos of stars and look at the results.

But how can you determine if one of your lenses is better or worse than another? You’ll need to have a systematic way to compare them.

You’ll want the stars to be close to the same place in the frame for each test photo. Since the stars are constantly moving, you’ll want to take all your test shots in a very short time period. If you have multiple lenses, that will not be easy to do. Especially for longer focal lengths. You could mount your camera on a star tracker like the Sky-Watcher Star Adventurer, but not everyone has a tracker to do that.

What’s a solution to this problem? Check out the following blog post and YouTube video that shows just how to do that.

Matt Hill’s Coma Testing Procedure

Matt Hill of National Parks at Night wrote an excellent article showing how to check your lenses for coma called “Getting the Best Star Points for Astro Landscapes: How to Test Lenses for Coma”.

The first step Matt suggests when testing lenses for coma is to get a notebook and a pen. Then make a table to record the camera photo frame numbers and exposure information for each of your lenses. That is especially important if you have any of the manually operated lenses that are popular with night photographers. Since these lenses don’t have any electronics in them, they cannot send EXIF data for your camera to record.

Table Your Data

Construct the table so that your lenses are in the column headers across the top of the table. Then label the rows with the f/stops that you will use for your tests. Matt’s lenses ranged from f/1.8 to f/2.8, so he made rows for 1.8, 2.0, 2.8, 4, and 5.6. He shot to f/5.6 as that’s where most cameras show no coma. You might want to do half-stops as well.

For lenses that don’t have an aperture setting listed in the table, write “NA” into that table cell. You don’t want to accidentally put data into that cell when you’re out in the dark! You can make this table before heading out to the field.

NPF to Rule Them All

PhotoPills app Spot Stars Calculator - for Testing Lenses For Coma
PhotoPills app Spot Stars Calculator

Matt then added a couple of rows to record the NPF Rule setting for each lens. The NPF Rule is a more accurate way to determine when trailing will appear in un-tracked star photos. It gives a much more accurate maximum exposure time than the old 500 Rule did, as it considers both the format of the camera sensor and the pixel dimensions.

To find out the NPF time, Matt used the PhotoPills app. If you don’t have the PhotoPills app, which I do recommend having, PhotoPills has a web page to do the calculation for you.

Matt recorded the “Accurate” and “Default” exposure times for each lens. According to PhotoPills, the Default setting gives barely noticeable star trails while the Accurate time is said to be “useful for large prints” and is the most prolonged exposure that gives sharp results. The default setting exposure time is about twice as long as the Accurate time. When testing lenses for coma, you’ll want to use the Accurate time, to get the roundest stars. Matt shot his tests at both settings to compare the results and to satisfy is curiosity.

Into the Night

Now that you have your table laid out, it’s time to head out and make your test exposures. Matt waited for a night with a new moon, so he had dark skies. He wanted to be able to see “the faintest of stars.” He also traveled into the countryside and found an open area with no obstructions in the sky overhead.

Matt also found a location where he had a light on the horizon about 1/4 mile away. He did that on purpose to allow him something that would be easier to focus on than a star. Once he had his focus set, he took a photo of it as a record of his focus. He then examined the shot to make sure the terrestrial light and the stars were in focus.

Matt left the camera with the horizon positioned about midway into his frame. You may want to turn your camera upwards to the sky. Doing this would give you a chance to see the lens performance in each of the corners. Try to get a brighter star near the corner and narrow edge of the frame.

When testing lenses for coma, you’ll want to set your camera to ISO 6400 and use the “accurate” NPF exposure time for your camera/lens combination. That way, you’ll be able to change the aperture settings and still know stars will show up in all your exposures.

As you take your test photos, write the file number of each photo into the cells on the table. When you’ve taken all your photos, head back inside and load them up into your computer to make a comparison of each frame.

Time to Compare

Make sure you check out Matt’s blog again to see how he made his lens comparisons. He combined all the images into a 100% crop into a PSD file. (You can even download Matt’s PSD to see the results of his lenses.) You can zoom in and peek at individual pixels in the images to your heart’s content! Matt includes a star from the upper left corner, the left edge, and the center of each frame.

What you want to look for in each image are any stars that are not sharp and round. When zooming in, you’ll see any coma quite easily. Coma shows the most in the corners of your images. It’s the oddly shaped distortions that are typically caused by coma that should interest you. In some lenses, coma looks like a flattened cone, like in the ray-trace coma example shown above. In others, the distortion will have a bird-shaped pattern. Matt points out the color fringing on the distortions can be fixed easily in Photoshop or Lightroom.  

Matt tested the Samyang MF 14mm f/2.8, the Venus Optics Laowa 15mm f/2.0 FE Zero-D, the Viltrox 20mm f/1.8 MF, and the Zeiss Distagon T* 15mm f/2.8 ZF2, all in Nikon Z-mount except the Zeiss which was in F-mount. I’ll not spoil which lens performed best, as you should go to Matt’s blog or watch his YouTube, where he discusses it. But I was surprised by his results!

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Testing Your Lenses for Coma

Hopefully, your lenses will perform well near wide open. If they don’t, you’ll be able to determine how much you need to stop your lens down. Keep in mind, if you are doing a panorama, you’ll not need to use the corners of your images as you overlap exposures. So, be mindful of that as well.

Also, make sure you look around the National Parks at Night website. Matt Hill and the rest of the National Parks at Night group (Gabriel Biderman, Tim Cooper, Lance Keimig, and Chris Nicholson) have a lot of great content there that’s sure to be of interest to Milky Way Photographers.

Your Lens Test Results

Have you tested your lenses for coma? Let us know. I’m planning to check mine out when the weather gets a little better! I have a general sense of where they fall, but it will be useful to test them all together.

Other Articles of Interest

Focusing stars is an important skill for Milky Way photography. Learn more here:
Milky Way Photography for Beginners – Camera Techniques

If you have images with coma (or astigmatism) that you want to remove, learn more here:
How to Remove Coma Aberration From Your Astro Images

kdk 1/13/20

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