This modification - shaving 1-2mm from the rear element after removing the protective baffle - is all that's required to use the Leica 19mm on the 1Ds I and II camera bodies. However, using the same lens on a Canon 5D will require further modification: generally more shaving. The exact amount will vary from camera to camera because of the variable length of the 5D mirror. If you run out of metal - or courage - stop shaving the lens and start hacking the mirror! The following is extracted from Arne Harving's forum entry:
One needs to remove a slice of the upper third of the protruding ring that protects the lens barrel proper (which holds the glass). First find a nice, clean file (for metal) and fasten it in a vice so that it protrudes 1-2 mm above the top of the vice. Then tape over everything on the back of the lens except this protective ring (I used masking tape because it removes easily and leaves no traces) paying special attention to cover all holes leading into the lens. Focus the lens to the minimum distance and place a tape over the lower 2/3 of the ring in a horizontal direction. Grind off the upper, untaped part of the ring by moving the lens over the file slowly with light force angling it about 30°, using the horizontal tape as guide. Have a vacuum-cleaner ready (with the narrow mouthpiece!) for frequent removal of filedust. When the ring is finished, focus the lens to infinity and remove in the same manner what small part of the lensbarrel that now protrudes beyond the cut on the ring. This is the only really critical procedure as the rear lens element can scratch against the file, so proceed with caution.
Clean the lens and apply two coats of matte camera paint and voila! it's finished.
So how much do you need to remove? The distance from the lens flange to the mirror-frame when in the upper position is slightly more than 7,5 mm. The protective ring of the lens protrudes almost 8,5 mm into the body so about one mm could be sufficent, but I suggest at least one and a half mm on top and tapering out to nothing one third down the ring.
When reading the above it sounds perhaps complicated, but in fact the whole procedure including setup/cleanup took less than one hour. The lens can still be used on Leica R cameras as before and can even be put on a table without tumbling over.
For those who might attempt this modification, may I also suggest you do an incremental distance test to establish correct infinity focus...
Jack Flesher's Modded Leica 19mm
Here's Guy Mancuso's.