d. In theory the point of focus should 'pop' in and out more, but in practice there is only a subtle difference.
In the days of manual focus slrs only the very cheapest (Zenit) had a plain ground screen with no other focusing aid such as microprism or split screen.
I put those lenses on my 5DIII and I miss focus 80% or more of the time when shooting wide open. It's probably the only reason I haven't upgraded my 5DII. The regular screen looks so grainy on the 5DIII it drives me nuts, not to mention that it doesn't give me an accurate bokeh preview.
I suspect this may be more to do with the transmissive/translucent LCD displayed over the focusing screen on the 5Dmkiii. (Take the battery out of the camera and the screen will go dark). It is one of the reasons I changed from
Nikon in 2005 when I did much manual focus. Now Canon are fitting them
. However since the advent of AFMA and BBF I really dont manual focus much now.
I agree with you on interchangeable screens for the mkiii, if for nothing more than to be able to see real dof and clean the inside of the thing !!
I suspect the best current camera to use the Otus on without an LCD overlay would be the 6D with 's' screen.
Now I know that by chance eml's son has one that his Dad could try.................