I've done some preliminary resolution testing, may make a real post with crops etc later. Anyway, using the USAF test slide I tested both 7D and 5Dmk2 at 1:1 macro distance with the latest Sigma 150mm macro lens. The lens holds up corner sharpness very well, but a little CA correction is useful.
Some numbers:
- 5Dmk2 at f/8 - ~3600 ppi, 91% of pixel resolution.
- 7D (APS-C) at f/5.6 - ~5100 ppi, 85% of pixel resolution.
- 7D at f/8 - quite noticable hurt by diffraction - ~4400 ppi, thus only 75% of pixel resolution
To get that high pixel resoultion absolutely top notch demosaicer algorithm in the raw processor is required. Only very small reduction of sharpness in this lens corners.
Compared to other tech:
- Flatbed scanners - Epson V750 Pro ~2300 ppi - only 40% of pixel resolution
- Film scanner 35mm CanoScan FS4000US - ~4000 ppi ~100% of pixel resolution
- Film scanner medium format Reflecta MF5000 - ~3050 ppi
I have a CanoScan FS4000US so I have used that as reference. I'd use the FS4000US for 35mm, or stitch/HDR with the 7D at f/5.6. I've improved my setup and technique so for the 22x15mm APS-C area the short f/5.6 DoF is manageable (one need to be extreeeemely thorough when focusing, I refocus for every repositioning, using precise leveling head and focus rail in collaboration), so I can do 5900 ppi digitalization with 5100 effective resolution. -3/0/+3 stop HDR on the 7D gives nice dynamic range, exceeding the film scanner and should be as much as one would ever need. Without HDR film scanner wins clearly, and I'd say it is to poor to use in prints. The greatest disadvantage of the FS4000US is that it is hard to get 100% of the picture area, one easily loses 0,5 mm or so top/bottom. When stitching one gets a margin.
For medium format the 5Dmk2 at f/8 plus stitching and HDR is very competitive compared to alternative tech. Flatbed scanners just suck, and the affordable and some fairly high end scanners too only do ~3000 - ~3200 ppi, sometimes with limited dynamic range. The stitched and HDR-merged result exceeds in resolution and DR. I've just shot a 60x45 medium format slide with the 7D, but that's really tedious - 16 frames need to be stitched, with the fullframe one can do it in 4, or 6 to get a bit more overlap. Most time during shooting is verifying and adjusting focus after repositioning.
The conclusion is that I think using a macro lens one can achieve better results than most think possible, but one needs a very sharp macro lens, small pixels on the sensor (to get the resolution) and very carefully executed technique. The next step up in quality seems to be a drum scanner.