21mm Top Guns

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

Performance at f8 (Zone A)

Nikon 17-35mm at 21mm / f8 (centre)	Canon 17-40mm at 21mm / f8 (centre)	Zeiss Contax 21mm at f8 (centre)
4.8 points	4.9 points	5 points

As you might expect, differences are flattening out by f8 . . . but the Zeiss 21mm not only resolves more information at this aperture, but it draws a more solidly etched, believable image. This combination of high accutance and vivid plasticity is very distinctive: the Canon and Nikon captures look like images shot with high quality lenses; the Zeiss capture is like being there.

Performance at f8 (Zone B: Left Side)

Nikon 17-35mm at 21mm / f8 (mid-frame)	Canon 17-40mm at 21mm / f8 (mid-frame)	Zeiss Contax 21mm at f8 (mid-frame)
4.9 points	4.9 points	5 points

Again the Zeiss retains its resolution advantage. There's very little to separate the Canon and Nikon zooms here.

Performance at f8 (Zone C)

Nikon 17-35mm at 21mm / f8 (corner)	Canon 17-40mm at 21mm / f8 (corner)	Zeiss Contax 21mm at f8 (corner)
4.4 points	4.2 points	4.8 points

It is sometimes said that the distortion of circular objects in frame corners is a function of focal length. As you can see in these captures, all at 21mm, that's not entirely true: many wide angle lenses suffer from negative (barrel) distortion in the frame centre, but positive (outward sweeping) distortion in the corners. Generally, these are the ones to avoid if distortion is germane to your subject matter. Many of the old-school Nikon designs were either very close to neutral (15mm and 28mm AIS) or had simple barrel distortion (without the positive kink in the corners). Some of the more recent designs (like the 14mm f2.8) are much more troublesome.

If you want critically sharp corners from a 17-40L, you'll have to stop down further than this . . . .