The Sigma EX 12-24/4.5-5.6 DG did seem like a dream lens from some reviews I saw and was very affordable too. It is great to experience 12mm focal length through the viewfinder and it seems like the perfect UWA. However, it is the only lens in my arsenal that isn't my real buddy. I do understand the technical challenges with making such a wide lens, but it is simply too much of a gamble to use it.

- It flares terribly easily. Any lamp, ray of sunshine, headlight or window is a big risk
- The CA/purple fringing is very significant
- Sharp light sources yields halos and/or over-pronounced "ray stars"
- The color and contrast is really mediocre - "lack of crispness"
- The resolution is poor
- It is slow (4.5-5.6), yet soft wide open. This means that you de-facto always need a tripod for interior work, as it is difficult to get good flash coverage with such a UWA lens

In summary - it is horrible. I may have too high expectations, but the Sigma is not fun to use (well to USE it is fun, but reviewing the results isn't). Although not that wide, my Canon EF 24-70/2.8L and 16-35/2.8L are both great and it gives predicable results all times - great colors and good sharpness and detail, even wide open. The TS-E 24/3.5L is a bit hard on purple fringing, but it is great for architectural work.

My intention is not to be "Canon L" snobbish - there is more than a 1:2 price difference to the EF 16-35/2.8L, but the quality difference is just so enormous even though the 16-35 is by no means any of the better Canon L-lenses.

A few samples and comparisons (when applicable + samples are often taken at at different times) below, which I find representative for the Sigma's shortcomings. No postprocessing, including sharpening is applied (except for conversion CR2-JPG in C1 3.7)
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