Originally posted by Fogel70 The only thing the size of the mount is limiting is maximum aperture size, not how much the image circle of the lens will cover.
FI there are FF lenses with rear lens element smaller than 10mm, and they still cover full FF image circle, and they probably also cover enough for SR.
As I mentioned before, this is not true. Generally speaking, wide-angle lenses can be built with small rear lenses but telephoto lenses can't. BTW, there is a reason why camera systems with larger image formats do have larger mounts …
Quote: And for normal image stabilization, +- 0.5 mm of sensor movement will be needed for FF.
Don't know what you call “normal image stabilization”. The Pentax APS-C sensors can move by ± 1 to 1.5mm; a larger format needs larger movements as the same field of view is projected onto a larger area. Therefore the pixel movement is amplified by the crop factor and, accordingly, the sensor has to move more for the same effect: ± 1.5 to 2mm.
It's somewhat unfortunate that a larger sensor, which is heavier anyway, needs larger movements, and explains why there is no IBSR for the 645 format (although the lenses do cover a large image circle of about 75mm while the sensor only measures 55mm diagonally).
I really don't want to spoil the fun, but we shouldn't entertain any hopes that Pentax can do anything that is physically impossible. And whine if they don't.