What is coma?
You might guess that it is a medical issue, such as a person with a profound unconsciousness caused by an injury or disease. But no surprise, in this case, you’d be wrong. What we are talking about here is the photographic definition of coma which is: a spherical lens aberration in which the image of a point of light is rendered as a comet-shaped blur. A lens aberration, of course, is any imperfection in the way the lens focuses light. (There are a number of lens aberrations but this article is mainly limited to coma).
Coma is not to be confused with chromatic (color) aberration, the colored fringing which is most visible in high contrast areas of an image, often where dark elements meet a light sky. There is another aberration somewhat similar to coma called astigmatism which stretches points of light into short straight lines. My guess is that most photographers have not seen or worried much about coma or astigmatism, with the possible exception of those who specialize in photographing the Milky Way (astrophotography) or other night landscapes. But these aberrations should be of interest to anyone who is looking to invest in a quality, large aperture wide-angle lens that would work well for both astrophotography and general photography.
A point of light should appear as a sharp point at all locations in an image, but off-axis point sources are susceptible to coma (and astigmatism) which causes blurring and deformation of a point of light. Coma (and astigmatism) occur mainly near the edges of wide angle lenses, especially when used at their largest apertures. Coma and astigmatism are unique in that their effects cannot be corrected in post processing like chromatic aberration or vignetting which are easily fixed. So, if you have a lens that exhibits bad coma, you are pretty much stuck with it, although stopping down to a smaller aperture should reduce coma and likely eliminate astigmatism. The problem is that if you are photographing night landscapes, you probably wish to use the largest aperture possible to reduce exposure time and allow use of a lower ISO.
The solution? Select a lens that is well corrected for such aberrations for this type of photography. Don’t know if your lens has bad coma? You might be able to find a reference online that has tested your particular lens for this problem, or you may want to test it yourself at various f-stops by photographing a night landscape containing points of lights. Unfortunately, lens manufacturers don’t provide much information on this but if their lens incudes an aspherical element(s), then it probably has some correction for coma. Otherwise, if you are looking for a good lens, you will probably have to rely on tests or testimony provided by others. The limited research I have done on this indicates that some of the best lenses for astrophotography are manual focus prime lenses, some of which are substantially cheaper than the high-end general purpose lenses made by camera body manufacturers. Here are some of sites that you might want to check if you want more specific information on coma or lenses that would perform well for night photography.
http://www.lonelyspeck.com/lenses-for-milky-way-photography/
http://www.dpreview.com/forums/post/39709189
http://intothenightphoto.blogspot.com/2013/02/overcoming-coma-aberration-part-2.html