I have managed to shoehorn......no wait......drop in a Nikon FE focusing screen into my 5D Classic with no modification, AND manual focusing is spot on. I've been testing it with pre-AI Nikkor lenses on a Fotodiox black Nikon-EOS adapter and so far the results have been nothing short of owning a 12.1 megapixel Nikon FE.
The viewfinder is plenty bright and actually brighter than the Ee-S screen which was made for the 5Dc for manual focusing.
Lenses I have tested so far have been
50 f1.4 Nikkor-S
55 f3.5 Micro-Nikkor macro
105 f2.5 Nikkor-P
300 f4.5 Nikkor-H
I'll get some proper ISO 100 samples up later today, but us 5Dc guys finally have a split-image focus screen that isn't some ****ty Chinese version that doesn't meter correctly. 


