This thread is being discussed over at the LUF as well.
http://www.l-camera-forum.com/leica...0904-digital-back-leicas-m3-m7-technical.html
Andy Piper has posted some interesting points - how do you get around them?
Just so we're clear - the "door" on the back of Leica M's was never intended for accesorizing - it is there to aid in film loading. Note that there was a 25-year-span when "data backs" were all the rage. Yet Leica never produced a data back for Ms.
Now - technical realities:
1) Are we talking a cropped sensor or full-frame? If the sensor is cropped, the mask for the framelines must be replaced so that the framing matches the image area (a la M8). That requires major surgery, so it would mean no fast swapping between digital and film. Basically a one-time conversion to digital - or at least a 2-3 week delay every time one wanted to change from digital to film or vice versa.
If the sensor is full-frame, the framelines do not need to be changed, but there would need to be permanent major surgery to the film rails and shutter opening, cutting them away to allow the sensor (with its glass overlayers) to fit into the correct plane of focus. Perhaps in combination with surgery to the lens mount, countersinking it into the body a bit. The idea being to retain the 27.78mm register or flange-to-focus-plane distance. The second would also require some kind of adjustment to the location of the rangefinder cam/arm - probably a new arm with a 'step" in it to move the cam back a mm or so.
But in either case, realistically, the camera would no longer be of use for film work, without equally major sugery to restore everything to original condition each time.
2) Electronic communication. At the very least, there needs to be a triggering signal to let the digital circuitry know that an exposure is coming. This needs to occur before the shutter opens, so using the "X" flash sync won't work - it only fires AFTER the first shutter curtain is open. Probably the best technique would be to borrow from the Hasselblad CFV backs, and use shutter button motion to arm the digital circuitry. The M4-2 and M4-P have a small opening in the inner bottom plate (the one with the film loading diagram printed on it) for use with motors. A shaft from the shutter button leads to this opening, where its vertical motion can signal the motor (or digital back) when the button has been pressed and then released. Probably possible to retrofit this to other M models.
Ideally, there should also be ways to communicate other data from the meter, lens, and so on for the best possible digital image - the 6-bit lens coding for the M8/M9 being one example. But, as with the lens coding list in the M9 menus, this could probably be manually set directly in the digital back, along with ISO and shutter speed info, although it would slow down operation significantly.
3) back to the sensor. Again - what size (or crop)? For anything above a 1.5x crop (based on my experience with Epson and M digitals) you really need the proprietary Leica/Kodak sensor design with offset microprisms to avoid vignetting. A generic "SLR" sensor just won't produce acceptable results with anything wider than a "32mm" view, in either size. Unless one uses SLR lenses with their long back focus (and no rangefinder coupling). Presumably not what we are after.
If Leica is going to built this, fine. If a third party is going to make it, it is going to require either a lot of very expensive sensor development to replicate the Leica/Kodak sensor without violating patents - or living with a significant crop and buying off-the-shelf Sony SLR sensors (as Epson did).
Taking all of the above into consideration, a cropped sensor is about the only solution. As in the DMR, the Hassy V backs, and every other "back" produced to make digital cameras out of film bodies going all the way back (no pun intended) to the Kodak/Nikon DCS-100: Kodak DCS 100
4) Bulk - the DCS-100 is also probably a good example of how much bulk would be required. At the time, it actually required a whole shoulder-slung computer for image storage and viewing (note there is no LCD on the back) - but 20 years on, that could all be fitted into the camera, today. And, of course, we could do better than 1.5 Mpixels (!!)
So, if a 1.5x crop, a more-or-less permanent conversion (not a swappable back) and roughly the bulk of a DCS-100 (minus SLR-isms like the prism) is acceptable, we move on to commercial viability.
I would guess I could bread-board a working prototype out of an M body and a Nikon D300 for under $100,000. Essentially, it would be a 10-12 Mpixel Epson R-D1 in a much lumpier package. Most of the expense would be for a programmer to soothe the Nikon firmware so it doesn't panic when it finds most of its shooting data missing.
After that, and assuming the customer provides the M body, I'd expect production conversions to involve:
Buying a D300 (assuming one wants "new" electronics) - $1800. Maybe with a wholesale discount from Nikon for 100+ cameras - $1200-$1400. Maybe, if Nikon will sell partial D300 assemblies - $1,000.
Disassembly and rebuild of D300 parts to "fit" M body - $1,500
Partial disassembly of M body and frameline change - $1,000
Reassembly of "M300" camera - $1,500
New firmware installation: $500
Testing: RF calibration check, electronics test, etc. $1,200
Overhead/profit/whathaveyou: $1,500
So around $9,000 total + your old body. To get a lumpy Epson, not an M9.
Assuming my expense list is realistic (I may have forgotten something)
And I'd have to sell 75 conversions to break even on the initial prototype investment.
