JDM von Weinberg , Mar 17, 2010; 04:44 p.m.
I don't find the need for a RF type wedge on a 5D at all. The screen, certainly by comparison to the smaller VF on the xxD series, seems eminently easy to me to focus on, even with my tired old eyes. I have the default, the Ee-A, S, and D screens for my 5D and use the grid (Ee-D) screen all the time with a large number of manual focus lenses ranging from a Reflex-Nikkor 500mm down to a M42 wide angle.
First, I'd try focusing these lenses on the camera as it is.Then try one of the fine-focusing 5D screens made by Canon. They still sold, the last time I bought one a year ago, for around $35.
In any case, a "split" screen for a focusing aid has to be made with different angles on the mini-wedges for different focal lengths. The more "universal" they are, generally the less effective they are as focusing aids. In addition to the problem of half the wedge going black with the wrong lens mounted, the effective base distance for any wedge-type split focusing aid on a SLR is very small. I never much cared for them on 35mm film cameras, much less wanting to put one on a dSLR.