DA 12-24mm vs Sigma and Tamron 10-24mm Comparison
Image Quality: Angle of View
The main reason why you buy an ultra-wide angle zoom is for the extreme angle of view. Lens manufacturers often claim angle of view beyond what you can achieve in the field. The only specifications that really matter are the ones that will show up in your images, and the only way to truly compare a lens is to view the final product. Since it is not always possible to have all three lenses on hand at the same time, we have taken the liberty of displaying all three images so you can compare them together.
Pentax 12mm f8
Sigma 12mm f8
Tamron 10mm f8
Comparing the Pentax DA 12-24mm f/4, the Sigma DG 12-24 f/4.5-5.6, and the Tamron SP AF 10-24mm f/3.5-4.5 DI II ultra-wide angle zooms, we found the Tamron (as expected) to have the widest angle of view. This is due to this lens being 2 mm shorter at the short end. The lens specifications state the angle of view as 108.4 degrees at 10 mm.
The Sigma the Pentax are both 12 mm at the wide end and have about the same angle of view, about 99 degrees with the Sigma being slightly wider. The specifications for the Sigma only provide the angle of view for the full frame format, but the tests confirmed the expectation that it is like the Pentax on an APS-C camera. There is no little asterisk that leads to small print stating this in the specifications. The Sigma came in second by a smidgin, while the Pentax came in third.