It looks like a combination of several things. Either a bit of haze or over processing is probably responsible for the fuzz around the brighter stars. Then there is some trailing which is more obvious in the brighter stars but is there is the fainter ones too. If you're shooting stars off a fixed tripod you only have a certain amount of time before they trail. The time depends on the focal length of lens (longer lens = more magnification = less time before unacceptable trailing) and and where you're pointed (stars near the celestial equator move a lot faster than stars near the celestial poles). Then you have to factor in what size are you going to view the image at? The larger you intend viewing the more star trailing matters. Focus could be an issue too. A bahtinov mask is extremely accurate but the lens might not be long enough for it to work properly. Live view is a possibility if you can get enough magnification, though my lcd screen has spent so long against my nose everything looks fuzzy on it.......... Autofocusing on a very distance object (by day or by night if there's a distant light or the Moon is around) and locking the focus works pretty well for my 17-50mm lens.