The 5DIII does not have exchangeable focusing screens supported from Canon. I have previously tried the Eg-S screen, made for 5DII and 6D, but that is not a mechanical fit. I found an instruction video on youtube, showing how to install it, but it did not work well over time. After a couple of weeks I got a focus offset (appeared as a +75 AFMA offset). Using AF still worked, but the visual appearance in the view finder, when the subject was in focus, was anything but in focus.
I have since only used my 1DX with the Ec-S screen for manual focus lenses. I hardly ever use a tripod, so I must be able to focus through the viewfinder. I am not able to focus with handheld live view.
A couple of weeks back I ordered a custom made S-type focusing screen for the 5DIII, from Focusing Screen in Taiwan (www.focusingscreen.com
). It arrived today and I have installed it (only takes about 5 minutes) and played with it. Mechanically identical to the original and fit perfectly. Initial impression is very good. I have only used it indoors, in rather dim light, but my keeper rate (Otus 55mm @f1.4) is very high (still subjects) and equal to what I am able to get with the 1DX.
I have not done a very thorough testing of the need for exposure compensation. But as far as I can see from the images I have, there does not seem to be a need for any adjustments. But the viewer does get a bit darker, so it is best used with f2.8 or faster lenses. I tried the 600 f4L IS II, but (for dim indoor shots) then the viewer turned too dark. That was expected though.
So, for those of you who are happy with your 5DIII, but would like to also use manual focus lenses, this is clearly a good option.