Yep its quite a mouthful, but the Contax 17-35mm N Mount – EOS conversion lens offers very good wide angle performance on the Canon SLR. Its a big lens, that is no lightweight, weighing in at 900g or 2lbs! it requires 95mm filters. Optical performance is very good for a wide angle zoom, it is not perfect, with somewhat softer corners at wide apertures. With one or two exceptions this type of wide angle zoom performancer is the norm for almost all manufacturers.
Lets talk about the good, and there is plenty of that Zeiss look! The lens renders with that 3d zeiss look that we all like! It shows great macro contrast, good shadow illumination and reasonable chromatic abberations or CA for short. CA is almost a non event these days, and lightroom deals with very well. Wide open at 17mm you can expect a full stop of fall off (vignetting) this decreases to about 2/3rds of a stop at F8. Fall off at 35mm is less, with about 2/3rds of a stop wide open and a little less than 1/3rd of a stop at F8.
Ditortion ranges from mild barrel (4%) at 17mm to slight pincushion at 35mm (1.5%). 24mm shows almost no distortion at all.
Colour transmission is good with good excellent contrast. Again we see the benefit of the T* coatings working here.
Functionally, its a big lens on the hand, its easy to use. The zoom ring turns the same ways Canon zooms, left to right, 17mm to 35mm. Manual focus is available. The Conurus conversion is a wonderful example of high quality engineering and great execution. The lens simply performs as if it was a Canon lens. You get autofocus, and auto aperture. Stop down preview is available too.
Here is a detailed test that I did several years ago comparing an early copy of the lens to a Zeiss 15mm and Canon 16-35mm V1 The conurus firmware updates have improved focus accuracy substantially since this test was done.