The Sigma 30mm f/1.4 EX DC is a fast lens for APS-C crop DSLR cameras. I got this lens primarily for photographing the night sky and it really isn’t a great choice for that due to the coma found on the corners of photographs. Coma is where stars form little checks, almost like triangles. If the images are not enlarged much, it doesn’t detract from the quality of the photograph. However, if you do enlarge they are distracting for anyone who knows anything about photography. The average person probably will never notice.
Other than the limitations of astrophotography the lens is good. It is capable of good photographs in low light, and the 1.4 aperture also allows for playing around with a very shallow depth of field. I didn’t have a lot of use for such a lens at the time, so I saved a considerable amount of money going with this sigma compared to a Sony.