For architectural applications – or indeed any use where straight lines must be naturally rendered – distortion control is critical. Sadly, it is very difficult to design even a retrofocus 21mm lens with this in mind. Compare the Zeiss Biogon with the Zeiss Distagon and you’ll see what I mean. One may as well ask for the moon on a stick as to request a 14mm that keeps horizontals and verticals true.

To see how these three compare, here’s the window from the top left corner. Great pains were taken to ensure that the camera back was absolutely parallel to the wall in order to really see what the glass was doing. These samples all at f16…

Canon 14mm at f16 (corner). Moderate waveform distortion
Nikon 14mm at f16 (corner). Horrific, terrifying waveform distortion
Sigma 12-24mm at 14mm/f16 (corner). Ruler-straight perfect. Zero distortion.

Even at close range (these tests were shot at a distance of about 75cm) the Sigma showed no more than a trace of waveform distortion.

Mouse over each of the Sigma 12-24mm frames to see how the same scene was seen by the Canon 14mm L and Nikon 14mm EDIF.