21mm Top Guns

Canon 17-40mm f4 L v Nikon 17-35mm f2.8 v Zeiss 21mm f2.8

Performance at f4 (Zone A)

Nikon 17-35mm at 21mm / f4 (centre)	Canon 17-40mm at 21mm / f4 (centre)	Zeiss Contax 21mm at f4 (centre)
4.3 points	4.5 points	4.9 points

The Canon 17-40mm L makes quite an entrance at f4; I don't know what they've been doing to them since 2006, but the centre frame performance from the latest f4 L is outstanding: at least as good as the Canon 16-35mm and even shading the Nikon 17-35mm f2.8 despite the one stop advantage. It bears scrutiny even alongside the Zeiss 21mm which is utterly sweet one stop down.

Performance at f4 (Zone B: Left Side)

Nikon 17-35mm at 21mm / f4 (mid-frame)	Canon 17-40mm at 21mm / f4 (mid-frame)	Zeiss Contax 21mm at f4 (mid-frame)
4.5 points	3.9 points	4.2 points

Here's where the Nikon throws a spanner in the works: it's distorted field of focus allows it to be considerably sharper in Zone B than Zone A: sharper even than the Zeiss 21mm in this capture. I can't account for the anomalous behaviour of the Distagon in this frame: everything points to a score of closer to 4.8 than 4.2. At full aperture the Canon 17-40mm predictably declines in resolving ability as we move across the image circle.

Performance at f4 (Zone C)

Nikon 17-35mm at 21mm / f4 (corner)	Canon 17-40mm at 21mm / f4 (corner)	Zeiss Contax 21mm at f4 (corner)
3.3 points	3.4 points	4.1 points

Naturally the Canon has the worst vignetting at the extreme corner, but surprisingly resolves more information than the Nikon despite its one stop handicap. Note also that the Zeiss 21mm still suffers from vignetting at f4. However, its resolving power is unmatched. If only the distortion was less problematic . . . .