Originally posted by Jewelltrail When I was reviewing my options: either the Sigma 10-20 or the Pentax Da 12-24 I took a good deal of time. There were many factors which pulled me in either direction. I did give the Sgma at least +1 for 10mm, but could not give this to the Da for 20-24mm since my Tamron 17-50mm already covered me from 17 to 24mm. This proved to weigh, decisively, in favor of the Sigma, along with Sigma's excellent handling of CA.
focal length needs is another deciding factor in buying a lens. on my part, 12mm is overly sufficient for outdoors that I didn't have to use the wide 12mm that often. I usually play along 14-18mm and thus hitting the sweet spot of the lens. the 12mm is sufficient and never needed anything wider for my close corner indoor shoots. so basically, I needed the added versatility and very useful extra reach of 24mm more for filling in the gap at that focal length as compared to something wider. and I love the results of the DA12-24 at 24mm. if I went for the Sigma, I would had definitely looked for a very good 24mm prime lens that would had cost me an additional $400-$500, thereby spending around 1k as compared to buying something half of 1k. I went for practicality and at the same time, a much superior overall IQ and versatility in 1 lens.
as far as cons are concerned, the 3 lenses have their own share of weaknesses. distortion for the Sigma, ugly colour rendering for the Tamron, and CA for the Pentax. so choose your own poison.