Coma is named for the shape of the error that we see when the effect is present. The image point of a source is spread over a comet-shaped region.
Coma is related to spherical aberration, in that both are caused by the shape of the lens. However, coma (and the rest of the aberrations that we will look at) is not present in the centre of an image, but gets progressively worse as we stray from the centre. We saw previously that spherical aberration and chromatic aberration appears even along the optical axis of a lens.
Below, we have carefully drafted the loci of the various rays from the source point. (The drawing was generated using Snell's Law with nair = 1 and nlens = 1.5.) The source point is directly above the optical axis, so the images are directly below the optical axis.
In all our diagrams so far, the lenses have been "2-dimensional;" we have not considered rays that have a horizontal displacement from the vertical plane going through the optical axis. When we do consider such rays, the image of a point looks something like this:
Coma is generally fixed using a second lens in conjunction with the first.