In optimal world I would buy a lens that is good in every way. But I'm on a budget and have to find a balance in quality and price. I'm looking for a lens for star photography, so in my case it will be a wide angle lens with good characteristics for capturing points of light in a dark sky. What would those characteristics be?
So far I've learned about coma, which seems like a thing to note when comparing different lenses for star photography. It would be quite disappointing if stars turned into long teardrops on the edges of a photograph even when every precaution was taken to make all stars sharp round points of light.
Vignetting would be an example of lens characteristic not so important in this use.
Many characteristics of a lens turn better when aperture is slightly stopped down, but I suppose I'd be wanting to use a lens with wide open aperture when trying to capture the stars in a dark night sky.
I'm not asking for lens recommendations. But, as an example of what kind of lenses I've been looking at, I name Samyang 14 mm f/2.8 UMC manual focus lens. Its price level is $350'ish in Amazon and seems to run with name Rokinon too. It seems to be reasonably sharp even when wide open - my camera is an APS-C sensor camera.