The Pentax 645D jumps straight to fourth place in DxO's overall rankings, comparing very favorably with significantly more expensive models from the likes of Leaf, Mamiya, Phase One and Hasselblad. As might be expected given its murch larger sensor, it also bests the majority of 35mm full-frame DSLRs in the process, with only Nikon's mighty D3X yielding a better score among 35mm models. Among medium format cameras and backs, only the Phase One P40 Plus and P65 Plus yield better scores, which perhaps isn't surprising given that the 645D costs half as much as the P40 Plus, and just one-quarter the price of the P65 Plus. Of course, for studio use the 645D is still at somewhat of a disadvantage given its lack of tethered shooting capability and an ISO 50 option, but for its intended use as a weatherproof medium format camera for the outdoorsman, the 645D looks impressive indeed. More info can be found in DxOMark's Pentax 645D analysis. The comparison to the nearest-scoring medium format models -- the Phase One P40 Plus and Hasselblad H3DII 50 -- also makes for interesting reading, as does a comparison with the highest-scoring 35mm DSLRs from Canon and Nikon.
The Canon-mount Sigma 17-50mm F2.8 EX DC OS HSM lens, meanwhile, looks to compare well to Canon's more expensive EF 17-55mm f/2.8 IS USM model, with similar resolution and overall performance, less vignetting, but stronger chromatic abberation. Finally, the Sigma 70-200mm F2.8 EX DG APO OS HSM lens in Canon-mount form likewise performs well against Canon's similarly priced EF 70-200mm f/2.8L IS USM model, with slightly better performance on an APS-C camera body, but being bested slightly when used on a full-frame body.
More details can be found on the DxOMark website.
Pentax's weather-sealed medium format 645D DSLR.
Photo provided by Hoya Corp.