People who photograph starscapes know all about lens coma, even if they don’t know it by name. Unless you have some high-end optics, this is probably one of your bigger obstacles against capturing distant lights as pinpoints. Especially in the frame corners.
Coma is confusingly defined as “a variation in magnification over the entrance pupil”. It means that off-axis light rays (corners of the frame) are focused to look similar to a comet with a tail or a teardrop. Nearly all lenses have this defect to some degree.
Keep in mind that optics almost always exhibit a mixture of defects. You probably won’t see a clear teardrop-shaped defect.
This article shows you how easy it is to evaluate coma. It’s cheap and easy to do.
My go-to lens for photographing stars has been the Tokina 11-16mm f/2.8 DX. If you stick with 16mm, you can actually get full-frame coverage (e.g. Nikon FX sensors) with this lens. There is a price for doing this, however: pretty bad coma in the corners.
If you want to see just how bad coma can get, and if you want to see how different aperture settings affect it, then this test is for you.
Coma defect ray trace
"Pure" coma photo
Tokina 11-16mm f/2.8 DX
What materials do you need to test coma?
I use a small and cheap laser pointer (literally only 2 bucks) to substitute for my ‘star’. You want a really small and intense light source for this test, and lasers are just the ticket.
A cheap laser with some aluminum foil and lens tissue
As shown above, you will also need a sheet of lens-cleaning tissue and a piece of aluminum foil. I put the tissue/foil layers over the business end of the laser, and hold it in place with some tape. Note the little buttons above: the front one is the laser button and the rear one is for an LED light.
It should go without saying, but I’ll say it anyway. Don’t stare into a bare laser beam. During construction of this testing device, don’t point it at your eyes.
Foil and lens tissue taped over laser
As shown above, wrap the lens tissue and foil layers over the end of the laser, and tape it in place. I wrapped the tape loosely, so it doesn’t activate the laser beam button. I lightly pressed the foil over the dent where the beam is located, so I know exactly where to place a pinhole.
I used a small needle to make a tiny hole in the foil. You want this hole to be really small, so that it will act like a distant star in the sky.
Laser, hose clamp, and holding fixture
I got a little hose clamp to slip over the laser and its button. To activate the laser, use a screw driver to tighten the hose clamp until it just depresses the button. Loosen the hose clamp to turn the laser off again.
Verify the laser beam can be seen shining through the little hole in the foil before you turn it off.
Next, place the laser/hose clamp onto a holding fixture as shown above. I like this fixture for being able to position small stuff in nearly any orientation. I also use this fixture to hold small items for macro photography.
To perform coma testing, I put the test fixture at one end of a room and mount my camera/lens on a tripod at the other end of the room. Focus the lens at or near infinity, and set your camera on ‘manual’. Align the camera until the laser device is in the corner of the field of view, or wherever you want to evaluate coma. If you want to look at long-focal-length lenses, you’ll probably have to move outdoors and try this at night to get the distances you need.
Now, get out that screw driver and tighten the hose clamp to turn on the laser beam. Set your camera exposure and get into self-timer mode. Turn out the room lights and shoot away. Check exposure to make sure the laser spot clearly registers in the photograph.
Coma test results
I’m showing you the results of the coma testing, where the photograph is shown at 100% zoom (pixel level). The beam is photographed very near the frame corner. This test is using a DX sensor, which is only fair. FX sensor corners look much worse (although Tokina probably thought nobody would ever try it in the first place).
Really pixel-peeping at f/2.8 and f/4.0
As expected, the smeary coma looks worst at f/2.8. Just one stop down, and the f/4 shot has already nearly eliminated coma. By f/8, the laser dot is really small and nearly perfect. This lens is considered pretty decent in regards to coma, but it certainly isn’t perfect; on full-frame, it gets pretty bad when shot wide-open.
Since a lens aperture’s job is to cut off light rays from the outer portions of a lens, stopping down an aperture is the cure for coma.
As you can see, the test equipment I used is pretty simple. If you really want to see how a lens handles coma, this is how to do it.
Notice how you can dramatically reduce coma by stopping down a lens even a small amount. This is, of course, of little comfort to starscape shooters who are after those wide-open shots to drink up every photon of light.