I have started this thread as I wish to point out Mark's innacurate description of Leica's Autofocus system and also of ours.
Ill start with this quote...
An important thing that few photographers are aware of is that autofocus systems are not continuous; they work in zones. The Leica S2 has many more zones of focus from close up to infinity versus the competition. In other words, the S2 can focus more accurately at a specific distance.
This is absolutely not the case with Leica, Hasselblad, Canon or Nikon for that matter.
With a Phase Detection Auto Focus system we can measure any distance from near to infinity. There is a tolerance on the last measured focus error but this has still nothing to do with zones.
The only 'zoned' cameras were small compact cameras which used a different system to measure the approximate distance and then set the lens to one of a certain number of zones. Either a couple of zones or a few more.
Digital Compact cameras and I would guess cameras such as the Canon 5DMKII (in Live View mode) use contrast detection to determine focus. NOT zones!
If Mark had intended to mean Leica's last measured value was a higher tolerance than Hasselblad, Nikon and Canon then I would assume Mark would know our tolerance, Nikon / Canon's tolerance and finally Leica's tolerance. Please Mark, tell me your sources for these values - I have never seen these published anywhere?. I have not be asked by you for any information on the Hasselblad Auto Focus system.
Suppose I want to focus on a tree at about 200-300 feet. Using autofocus with a Medium Format camera, the system often goes into the infinity focusing zone. But, the tree is not at infinity. Add to this a high resolution MF back like a P65+, and the tree is guaranteed to look out of focus. Finally, focus shift could make matters worse if it goes the wrong way.
To be absolutely clear, this is not the way the Phase One 645DF works, or the Hasselblad H System.
Most lenses suffer from focus shift as you change the aperture. This is a problem, since autofocus systems focus with the lens wide open. Hasselblad makes a correction (in the H4 series) with a software model. The camera figures out the lens, the focusing distance and aperture, and just before the exposure it applies an algorithm that re-focuses the lens. The problem is that the algorithm is based on one "ideal sample" lens. Unfortunately, each real lens differs somewhat from the “model lens”, so the system is not totally accurate. The PhaseOne lenses in my experience seem to have less focus shift. As far as I know, PhaseOne does not make any corrections to compensate for focus shift; PhaseOne users simply have to live with it.
We have been making this correction since the H1 was produced. Again, please check your sources, or actually request the information.
How do you know by how far the model lens deviates from a produced lens? Again this is not published information and for what it is worth the model lens is actually very close to the produced lens, close enough for the focus correction to work and to be of value.
Have you made specific focus shift tests on the Phase One and Hasselblad lenses? It would not be possible to test the Hasselblad lenses anyway as the focus correction cannot be switched off. So please tell me how you measured this compared to Phase One?
In conclusion, as usual a review on Luminous Landscape which mentions Hasselblad contains inaccurate information. I am more than happy to answer questions, supply technical data or even proofread articles for technical aspects. You only have to ask.