Get Sigma 20mm f/1.4 DG HSM Art at B&H Photo.
This mundane scene is deceptively simple, yet amazingly fruitful.
It is hard to find a single scene that shows behavior in areas of sharpness and contrast, field curvature, color aberrations (lateral and longitudinal), vignetting, and flare, the point spread function, etc. And with challenging contrast and backlighting too. This scene does it all in one.
Includes images up to 24 megapixels as well as large crops, all from f/1.4 through f/13.
There is no question that the performance of the Sigma 20mm f/1.4 DG HSM Art ranks among the very best 20/21mm lenses available. It may well be the finest 20/21mm design out there (as of early 2016), and it is autofocus as a bonus. And it is available for Canon or Nikon or Sigma. Additional comments that add further perspective are found in the analysis.
The Sigma 20mm f/1.4 DG HSM Art is definitely an ultra-wide prime to have in the bag. The price should leave the mouth agape, given the performance.