(BTW, I still don't understand what the hell Ultra-Focus is all about)
Well I don't either so i asked a mate at 'blad and this is what he told me:
Part of the problem that was seen with adding digital backs to medium format bodies was an occasional discrepancy in focus calibration. Ie, the position where the photographer placed the focus (manually or otherwise) could then be seen to be different in the final image. The situation was worsened by capturing at maximum open apertures.
This could be cured by 'shimming' some digital backs (For example the Imacon Ixpress range) by the use of thin foils to bring the focus towards the correct position, albeit not always %100 accurately.
For many reasons accuracy of focus is much more critical on a CCD compared to film.
Therefore the first stage of Ultra Focus was to manufacture integrated units as 'one' as opposed to a production line of camera bodies and a line of digital units. Yes, the digital unit can be calibrated to a 'known' position but Hasselblad feels there is still an unacceptable tolerance with this method. Even a minimal difference in the thickness of the IR filter can change the focus position enough. (The optimum point of focus is on the surface of the IR filter, not on the CCD below).
Therefore the first stage of ultra focus was to assemble integrated DSLR's as 'one'. Therefore each camera was tested and calibrated as a single unit.
At the final stage the AF sensor in the H body can have an offset applied to be specifically calibrated to the digital unit. Therefore when working the camera in AF mode you can be assured of 100% focus accuracy. Equally using the camera in manual focus mode the AF sensor will also indicate when the subject is in perfect focus.
The next task to solve came with adjusting the AF depending on the working aperture used. Focussing is carried out with the lens (normally) fully open. Due to certain optical characteristics, the focus point can shift when the lens is stopped down to the working aperture. For example, the Zeiss CF 80mm lens showed this problem quite clearly at certain distances and apertures.
Therefore when working with an H3D the AF drive is again offset by the correct +/- amount based on the working aperture selected on the camera.
So Ultra Focus begins in the first stage of manufacture and ends in the way the AF drive is implemented.