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It is not an off brand - pentax was a well established brand back in the 35mm film era.
Another question: when the soon expected 50mp Nikon FX sensor appears will there really be any significant advantage to buying a whole new system just to get a larger sensor; especially when we are now seeing lenses like the new Sigma 50mm f1.4 Art appear? It just seems to me the "need" as opposed to the "want" for medium format is diminishing.
Pentax doesn't make sensors for their cameras, almost all of them are from Sony. As for the MF sensor, I think it is similar to the ones used by Hassy and Phase One, basically and off the shelf sensor. Pentax was already into medium format during the film days, so they already had lenses for the mount. MF is a totally different ball game, and has some different applications. Some publishers simply require their photographer to use Hassy for Phase One bodies.
Pentax's previous offering was only 14bit color like DSLRs which was one of the reasons it didn't take off with the MedFmt crowd and quite a few tests put the D800 performing the same. It will be interesting if it is 16 bit color (H5D-50C is) as that would be the cheapest 16bit camera to date.
Its not 16bit, but 14bit still. I was pretty disappointed by that, but the camera is more of a FX camera on steroids than anything else, so it still seems like it will be easily marketable nonetheless. Its also the first MF camera to shoot video, so thats a bonus.
"The 645Z finds itself amongst some very pricey competitors, both of which use the same 50MP Sony sensor (with no AA filter, as you'd expect)."