Comatic Aberration and Chromatic Aberration

These two phrases are pretty much guaranteed to raise the blood pressure of optical aficionados: Comatic Aberration and Chromatic Aberration. There. Did your blood pressure go up? Then it is likely you have dealt with one or both of these issues before… and it is likely that you do not need to read further! For those looking around the internet for an example of these aberrations, seek no more!

Let us start with an image. This shot is of the December sky taken through a wide angle 20mm AFS Nikkor 1:1.8G ED lens on a Nikon D-810. The images were raw NEF files without any processing (except resize), either on board the camera or using software. Click on any image to see it in larger format.

Taurus and comet Wirtanen
Taurus and Comet Wirtanen

The image is a pretty typical night shot: 10 seconds focused at infinity and using 5000 ISO at f/2 (a little stopped down). The constellation Taurus is dominating the right side of the image. There is an airplane top-center moving to the lower left. If you follow the airplane’s future trail it leads to a faint greenish fuzzy object, Comet 46P/Wirtanen. This image is reduced in size…. but upon close, full-scale viewing, this image displays two of the common issues that astronomers and photographers aim to rid themselves of. Funny thing is that this lens gets fabulous reviews on sites like Amazon, and when I complained about these issues I was actually chastised! “Are you kidding? This is such a great lens!” Well, no. It’s not, and for the price, it really should perform a lot better. Add to this story the fact that the Nikkor 16mm fisheye actually is BETTER than this 20mm lens, and there you have an argument to not buy this 20mm lens. So, read on….

May I present to you comatic aberration:

comatic aberration in star images
Classic coma on the images of stars. These should be small round dots, not winged things….

This aberration is off to the sides of the image, off-the central axis. The further from the center, the worse this aberration gets. Some systems sprout seagull like wings from stars. This lens sprouts more than that. Ugly. The cause of this problem is in the optical design and is usually found in parabolic mirror systems like Newtonian reflectors. Alas, it also happens here in lens designs.

May I now present to you chromatic aberration:

Chromatic aberration in star image
An example of chromatic aberration: note the violet halo surrounding the bright star. The violet wavelengths focus at a different distance than all the rest. The fuzzy green object to the left is Comet Wirtanen.

Chromatic aberration has been the bane of the optical world for a long time, starting with those who first pointed telescopes up at the stars (i.e. those like Galileo, etc). A single lens acts very much like a prism in how it bends (refracts) light. The angle of refraction has to do with the light’s wavelength, so not all colors of light will come to focus at the same spot. This is usually handled with complex, multiple-lens systems like Petzval lens groupings using unique glass recipes than minimize chromatic aberration. Well, this lens? It suffers. When pointing at a bright white star, this lens gives an image very much like that of a simple two-lens refracting telescope, what is called an achromatic refractor. Well, they are notorious for having a violet to blue ring of light surrounding bright objects… and halos of blue around the moon and Jupiter. Not fun. Nope. This is why we have monstrously expensive systems like apochromats and Petzvals. We are talking expensive!