For long time five microns have sort of been seen as a practical limit. A five my sensor on 55 by 45 mm geometry would yield 99 MPixels, so it's certainly possible.
Shrinking the size of the pixels puts increasing demands on the rest of the system, but less dramatically than one would think. Going from from 60 MP to 90 MP is a 22% increase in linear resolution, so optical alignment needs to be 22% better, lenses 22% better and so on. BTW, photographer needs to be 22% better, too.
Regarding price I'd suggest that the cost of a 90 MP chip is the same as for a 60 MP chip. Camera electronics would need to be 50% faster to keep performance at a given level.
If I recall it correctly, the P45 was 39MP while the P65 is 60MP, according to Michael Reichmann it was a significant improvement, but that improvement probably went beyond just increasing pixel count. If Phase wants to get P65+ owners to upgrade they need a a significant step forward in image quality.
On the other hand, Phase just got some new competition, the Leica S2 and the new Pentax 645. So Phase perhaps would be more focused on improving their camera platform regarding precision, autofocus and so on.
does anyone have an idea as to what lies ahead in medium format land?
Does Phase One have some surprises for Photokina?
Different points to consider:
a) Dalsa apparently is able to create medium format sized ccds with 92 MPx resolution. They already manufacture them for aerial photogrammetry and cartographic applications ...
Optical limitations. I just received a Schneider 43 XL and must say the performance is incredible. It is another league compared to the base offerings of hasselblad (currently am on a H3D-50 loaner till H4D-60 is finally shipped). Apparently this is the bleeding cutting edge of lens technology and one can see that in the results. I have never seen such sharpness out of a digital back, even applying a little bit of sharpening can immediately create halos and jagged edges, that's how sharp this lens is. But nonetheless even these lenses are are apparently conceived to max out at 5-6 microns. I'm confidend such Schneider lenses will resolve a higher resolution back but what about let's say some stock 80mm 2.8 lens that's been around for years with the same optical design? I'm afraid the optics aren't up to the task and creating a bigger sensor is out of question short of creating a new kind of bigger system and replacing the H-System alltogether.
c) R&D considerations, market considerations: People who just recently coughed up the money for a 60MP system would be pissed if there would be a 90 MP system around the corner in this short time-frame and which would cost maybe another 20 000 USD to "upgrade". I'd be seriously enraged if Hasselblad came up with a higher-res system just one or two months later after they finally have been able after more than a year! to produce a useable H4D-60.
d) Phase One hasn't had a new product for some time and they are perceived as the key innovators in the MFDB area ... they have to have something new to show around Photokina ...
So ... what will happen, anyone care take a look into the crystal ball?