Originally posted by lister6520 Allowing the rear element to be placed closer to the sensor only helps to make extreme wide angle lenses cheaper to build, it does not make them any better. The rear element as it is in today's designs MF mounts already allows the rear element to be closer than what is prefereable for a digital sensor so reducing the flange distance won;t really help. You will just end up with the rear element being ahead of the lens flange instead of behind it if one wants to avoid the problems of low angle of incidence on the sensor.
Current lens designs for MF digital backs proof you wrong. The Rodenstock 23mm/5.6 S-Digaron leaves 16mm from the rear element to the sensor surface - certainly not enough for the Pentax mirror. This lens in fact can be had with focusing mount and theoretically could be adapted to a mirrorless MFD of short mounting distance. Few of the lenses for MF digital backs, even the telecentric designs, would fit a digital SLR with mirror, unless they are of longer focal length, and with these designs compactness certainly is not an issue.
There are also issues of convenience. Returning to the mirror-up design in the 21st century would certainly have commentators and online blogs scratch their pixels. Moreover, saving weight and saving cost does make a difference when one considers the cost for these lenses (e.g. $5K for the Pentax, $8K for the Rodenstock). And the judge is still out as to which designs would be better. Neither you or I have designed the new generation of lenses for the putative mirrorless P645Dii.
As I said - I didn't propose that the new digital P645Dii should become a Sony A7r on steroids. I'd hope Pentax would do their homework and calculate some excellent wide angle lenses in advance. Perhaps they'll also add a global shutter or a front-end shutter to deal with the shuttershock issue the Sony suffers from. I bought a dozen adapters for the Sony mount, but last minute decided to wait for generation II of the camera body. These mirrorless camera manufacturers are incredibly creative and every generation seems to introduce a bunch of innovations.
I am also not proposing that the Pentax K-01 was the cat's pajamas. An unsuccessful design that didn't save weight and had few if any advantages. I'd hope Pentax learns from this and other mistakes and brings something truly innovative on the market, designed for the photographers, as they did in the past.
I simply felt intrigued by the news that Pentax/Ricoh would come up with something new and different for the 645Dii and I am hoping it's mirrorless. You don't. Let's wait and see.