DA 12-24mm vs Sigma and Tamron 10-24mm Comparison
Image Quality: Chromatic Aberration
Zoom lenses are great for convenience but are prone to chromatic aberration. Chromatic aberration occurs because the lens is not focusing the different wavelengths onto the image sensor at the same point. The results are usually seen as a purple fringe on the image. Three conditions will increase the likelihood of chromatic aberration: high contrast, wide open aperture, and using a zoom lens at its extremes. To decrease the chance of chromatic aberration avoid high contrast situations if possible, if these situations cannot be avoided, then stop down your lens and use zooms at their midrange setting.
In the picture below you will notice the right side of the palm is exhibiting chromatic aberration. Just like distortion, chromatic aberration can be corrected by post-processing software.
The Sigma DG 12-24 f/4.5-5.6 was the clear winner. Not only did it have very limited chromatic aberration wide open but it was consistent throughout the f-stop sample. The Tamron SP AF 10-24mm f/3.5-4.5 DI II displayed some chromatic aberration at f3.5 but improved significantly at f8 and f16. The Pentax DA 12-24mm f/4 did not do as well as the other two lenses, which was surprising. Not only did it have significant chromatic aberration at f4 but it did not improve much as the lens was stopped down.