The Contax system uses in-lens motors and rear/inner focusing whenever possible. So why is AF not blazingly fast?
First of all the Zeiss lenses are heavy
. The Zeiss lenses for the Contax don't weigh any less than those for the Hasselblad and have the same manual focus feel. They all have stainless steel barrels and very solid construction, which , in the case of the Contax, come at the expense of AF speed.
The second reason, according to Contax in an interview with Asahi Camera magazine of Japan, has to do with AF precision. Contax has made a firm commitment to take precedence in image quality, and this commitment prompted them to produce an AF system that optimizes precision rather than speed
According to Contax, they have optimized AF precision on three levels. Most AF SLRs on the market use a small number of linear or cross-type AF sensors, usually in a horizontal formation. Each sensor is made up of a few hundred pixels. The Contax 645 is the first AF SLR to use a 1/4 MILLION-pixel CCD for its AF system. The result is a true area sensor (i.e. no gap) with supposedly much higher AF measurement accuracy. Secondly, the AF motor in each lens is optimized for precision with extremely fine movement rather then speed
. Lastly, in the case of 220 type films, the vacuum filmback ensures higher film flatness.
How much better is the resulting AF precision is not known without some testing, but there are indeed some compromises as a result. The AF motors being optimized for precision, comes short in speed as I said. Also, the choice of 1/4 million-pixel CCD, which means a lot more information needs to be processed, slows down AF a little more and adds to battery consumption. (This has been verified by Asahi Camera.)