First, let's remind ourselves what the basic features of the EOSM are:
- Sensor photo detectors (Mpix) 18.63
- Resolution is 5280 x 3528
- Sensor size is 14.9 x 22.3 (mm)
- (Crop) Multiplier relative to a 35mm FF camera is 1.6
- Aspect ratio is 3:2
If we now use the EOSM-645 Adapter in full 645 mode, ie 8 images, taken in two rows, with the EOSM in portrait mode we end up with essentially a 4:3 aspect ratio and with an image with a 120+ Mpix image, ie 12400 x 9820 pixels, or some 41x32 inches at 300 dpi
As for setting up the shot, an acceptable 645 hyperfocal distance (for now ignoring diffraction and using George Douvos's excellent TrueDoF-Pro App) is to use a (defocus) Circle of Confusion of 47 microns at F/20-22, is just under 2m. Note we can use such small apertures as we are shooting medium format, albeit via 'sensor bracketing'.
However, we wish to do better than an infinity defocus blur of 47 microns, so we will half that to, say, 23 microns, giving a focus point at about 4-5m.
So what does a medium format image looks like if taken with an EOSM? Well here is the test image I just took, with the help of Magic Lantern, ie ETTR and Dual-ISO. OK, it's not meant to be submitted for a competition: by it does show off the format, ie it's a 4:3 image and has 12400x9820 pixels (albeit scaled here)