I will admit I don't use my super wide angle Sigma 12-24mm f4.5-5.6 a whole lot, but a recent visit to Casa Loma, the magnificent early 20th century castle in Toronto, seemed like the perfect opportunity to pull it out. This lens is the widest zoom lens in production for full frame cameras, allowing an incredible 120 degree rectilinear view.
Though this lens is fairly slow at maximum aperture, the ultra wide angle means that you don't have to worry too much about shake, even at shutter speeds as low as 1/8th a second. At this shutter speed at 3200 ISO, as room is quite dark. The other thing about this lens is that you dont have to worry too much about focusing - almost everything is always in focus.
A rather interesting quirk is that this camera lens combination tends to underexpose scenes inside (particularly those with a bright light source, like a window) quite dramatically. One photo I have to add +3 stops on exposure compensation to get the correct exposure. Outside there were no issues.
Overall it is a good lens, reasonably sharp and not too much distortion. The corner quality does drop significantly wide open at 12mm, but this is to be expected. 12mm does give an unusual perspective on an object, and for this alone having the lens around is worth it.