Autofocus Converted Contax N Test: 17-35mm f2.8

Contax N 17-35mm f2.8 v Nikon AFS 17-35mm f2.8

Performance at 17mm: f16 (centre frame)

Contax N 17-35mm at 17mm/f16	Nikon 17-35mm AFS at 17mm/f16
0 points	1 point

Performance at 17mm: f16 (10mm off-centre, left side)

Contax N 17-35mm at 17mm/f16	Nikon 17-35mm AFS at 17mm/f16
0 points	1 point

Performance at 17mm: f16 (upper left corner)

Contax N 17-35mm at 17mm/f16	Nikon 17-35mm AFS at 17mm/f16
1 point	1 point

At last the Nikon's corners sharpen sufficiently to be acceptable. This, combined with its greater immunity to diffraction effects, gives it all the honours at this aperture. Many Nikon lenses, and this one in particular, remain completely useable at f16 – an aperture at which many Canon, and even Leica, lenses are over the hill. Note also that the Nikon 17-35's lateral CA persists in the corners at all apertures down at 17mm.

So what have we learned? The wider the lens, the greater the curvature in the focal plane. Both of these lenses effectively have 'ripples' in their focal planes: waveform distortions of field sharpness that are quite unlike the conventional 'linear' fall-off in resolution one expects as the edge of the image circle is approached. What makes comparison so difficult in this instance is that the peaks and troughs are diametrically out of phase: where the Contax is at its best, the Nikon is at its worst, and vice versa. Borrowing the framework of an MTF chart, their performance at 17mm can be summarised thus:
Without wishing to come on all Open University, in ranges A and C, the Contax 17-35mm has significantly higher resolution, whereas across all of B the Nikon appears sharper. If we consider these lines as averaged MTF graphs, we wouldn't normally expect to see either a dip in centre frame resolution (as we saw with the Canon 24mm L), or a dip in the 10mm range. We would also expect these lines to flatten and pull together at smaller apertures, where diffraction effects limit the maximum resolution and stopping down raises the resolution of the outer image circles – but in comparing these lenses the pattern persists right up to f16. Picking a winner at this focal length is difficult: for resolution, they must be considered about even. But in all other ways the Contax N is better: slightly better controlled barrel distortion (see Part II), far superior flare resistance, much better CA control, and of course it does automatically what the Nikon can only achieve manually. We move on to 21mm . . . .