Again, a word of warning over the Sigma 18-35mm f/1.8 - infinity focus on that lens shifts slightly with focal length. The infinity mark and on any lens is never going to be
perfectly accurate.Manual focusing is essential for this kind of photography as there isn't a single AF system in the world that is up to the task of focusing on the night sky accurately. Prevailing weather conditions can effect focus shift to a surprising extent.
Personally I wouldn't reccomend the Sigma 18-35mm f/1.8 to a newbie.It is best to do this kind of wide field astrophotography with a prime lens of f/2.8 or faster and with a focal length shorter than 50mm as there are fewer variables to get in the way of producing a satisfying image.
I use the Pentax O-GPS1's astrotracer feature to shoot night sky images without trails.
Pentax K5IIS - Sigma 18-35mm f/1.8 @ f/1.8 18mm 30s ISO 800.
This image was made with the above pictures set-up.
Sigma 18-35mm f/1.8 @ f/1.8 35mm 30s ISO 800
However at the "long" end of the Sigma 18-35mm f/1.8 vignetting is quite problematic, Coma is also beginning to become an issue as well.
Last edited by Digitalis; 08-03-2015 at 04:10 AM.