Canon offered optional focusing screens for the EOS 5D and 5D Mark II. These included the Super Precision Matte Eg-S focusing screen. With the 5D Mark III in 2012 and the 5Ds/5Ds R in 2015 Canon began to embed electronics for information display directly into the focusing screen and thus they are not interchangeable. In fact, the viewfinder on such cameras is not usable when the camera is not switched on. Without power the optical viewfinder becomes semi-opaque.

One option you might consider if you can find one is the Canon Angle Finder C. It is fully compatible with the EOS 5Ds and 5Ds R. It has a selector switch for 1.25X or 2.5X magnification.

• At 1.25X the entire frame is still visible, but it's magnified over the size of the viewfinder viewed directly, so when looking through it, the viewfinder looks like it is 1.25X larger.
• At 2.5X the entire frame is not visible, but the center of the frame is magnified by a factor of 2.5X.

Of course it is an angle viewer, so you'd be looking down into the viewfinder at a 90° angle from the lens' optical axis. The Angle Finder C does swivel 360°. You can look straight down into it with the camera pointed forward. You can look forward into it with the camera pointed straight up or straight down. Or you can look into it from either side of the camera with the lens pointed 90° to your left or right.