I'm back with a mirco-review of this adapter for those interested:
(1) I'm using the adapter with Contax 140mm and 210mm lenses thus far on a Nikon D800. It works really well. Fit and finish are good and focus is both relatively fast and fairly accurate (very little focus hunting and sharp focus in the resulting pictures).
(2) Indeed, judging from my recollection as a long-time Contax 645 user, focus with these lenses on the D800 via this adapter is significantly better than on the Contax 645 body itself. As those of you who use or have used Contax 645 know, AF was always somewhat slow, lots of hunting in anything other than the brightest light, etc. Because I loved the lenses, I was willing to live with that (often just resorting to focusing manually). But on the D800 with this adapter, AF is *dramatically* better than on the Contax 645 body. I'd say it's at least as good as my non-AFS Nikon lenses are on the D800. (That doesn't sound impressive unless you've used the Contax lenses on the Contax body, where the AF performance was nowhere near as good as even a non-AFS Nikon lens on a Nikon body).
(3) As it happens, I also have a Leica S and the Leica-made Contax-to-Leica S adapter. The JAS adapter + Contax lenses on the D800 body also has significantly better AF than the same lenses used with the Leica adapter on the Leica S body. I did a fairly extensive back and forth last night since I have both adapters and both bodies, swapping the same lens on each body using each adapter. There's no question that the AF performance with the JAS+D800 is much better than with the same Contax lenses using the Leica adapter+Leica S. I was completely flabbergasted by this. The Leica S adapter is decent: that is, its AF performance is reminiscent of the performance of the Contax lenses on the Contax body, so I really can't complain (Plus I didn't pay full price for mine: bought it used). But compared to using these lenses with the JAS adapter on the D800, it's no contest. I'm not sure if this is because the D800 AF motor is better/stronger than the Leica S AF motor, or if it's because the JAS adapter is better than the Leica adapter, or a combination of both. But whatever it is, it's pretty impressive that some random guy (no offense, obviously a very talented guy) made an adapter that rivals or exceeds the adapter made by a multi-billion dollar company, for less than half the cost. (I realize I may be comparing apples to oranges, since the difference may well be attributable to the different AF motors driving the lenses, rather than to the different adapter, or at least not solely due to the different adapters.) It tempts me to lobby him to make a JAS Contax-to-Leica S adapter....
(4) The AF performance of the Contax lenses on the D800 body via the JAS adapter is at least as good in terms of speed and accuracy as the AF performance of the two native Leica S lenses I own on the Leica S body. My initial impression is that it's definitely better than the AF performance of the 70mm Leica S lens (with the old firmware -- haven't been able to update the lens firmware yet for that lens) and at least as good, if not slightly better, than the 30-90 Leica S lens (with latest firmware), especially in dim light. In bright light, they're roughly comparable, but in dim light, it's pretty compelling that Contax+JAS+Nikon D800 is faster and more accurate than my Leica S lenses on the Leica S body.
