« on: January 28, 2015, 02:46:10 PM »
I'll post a dissenting opinion.
I've purchased just about every screen available for the 5D2. I have tried precision matte screens, split focus, the original, microprism and a few others. The prices varied from $15 to $200.
The short version is that none allowed my to achieve critical focus with my Zeiss ZE lenses.
I then tried a Zacuto Z-finder Pro loupe and mounted it to the lcd on a clip-on frame (pn Z-FRM or Z-FRM32 depending on your camera.) I can nail critical focus almost every time. Yes, it's a little unwieldy but the camera is pressed against your eye and it's stable. The second issue is that this goes through batteries, so you need a few in your pocket. It does however work.
If you replace a focus screen, and you need very accurate AF you must use shims to adjust the position of the focus screen. Canon sells the shims you will need for your camera. There is a procedure in the service manual, or you can use trial and error. Some of the focus screen manufacturers include a shim, but it is no guarantee of accurate manual focusing with your camera, each camera can have a different variation.
Focus screen manufacturers usually purchase 1 series or even focus screens from MF cameras, and mill them to the dimensions to fit. This results in inaccurate exposures, so you need to add exposure compensation in many cases.