The EG-S is great and I leave it in my 5DMkII all the time. Viewfinder brightness is comparable to that of the 7D and I don't ever find the viewfinder to be too dark. IMHO these FF camera's should come with the EG-S as standard.
How does the EG-S screen work exactly? I'm only familiar with the split prism manual focus screen.
Is the EG-S difficult to install?
Thanks!
The normal EG-A screen has micro lenses that are aimed at the center of the frame to produce a bright viewfinder (but also too much DOF), the precision matte is more like ground glass and much more diffuse. I can't quite explain how it works optically, but when you look through the screen it is much easier to see what is in focus and what isn't. The EG-S effectively shows a shallower DOF.
This site ( http://www.focusingscreen.com/work/5d2en.htm ) shows how to change focus screens, I would recommend a Canon EG-S over a third party one as a third party screen may interfere with the metering accuracy of your camera (notice that you need to change a setting in the camera to account for the different screen in use).
The brightness will really drop off with slower lenses. If you have an f5.6 or f8 lens (or stop your lens down to that and use the depth of field preview button), the EG-S screen will be a lot darker than a normal focus screen.
The rougher, or more matte the focus screen is (such as an EG-S focus screen) the more it can pick up on the off-angle light rays, such as you'd get with the converging light rays from a wide aperture lens. If a focus screen is smoother, such as the stock screen, the artificial 'ground surface' of the plastic screen doesn't have sufficient depth or angles on it to pick up on such off-angle light. The bulk of the light it receives is the stuff close to perpendicular to the focus screen - which explains both why these stock screens don't get as dark when stopped down, and why they can't render the out of focus parts of the image as blurred as the sensor can. Somewhere around f4 to f2.8 is where a stock focus screen fails to show how narrow the DoF is. If you use an f1.2 lens wide open with a standard focus screen, the viewfinder will be very different to the final image.
In response to the original question, focus screens such as the matte EG-S or stock EG-A give the manual focusing user the equivalent info (albiet scaled down) of what contrast detect AF has - no indication of exactly how much it's out by, or any real info about which way it's out. Plus without resorting to magnified live view, you can't accurately see if its 100% there, even with an EG-S screen. But you can hunt around, finding where it looks sharpest and opt for the middle of that acceptable range, hoping for the best if you're only using the non magnified viewfinder. However, a split image focus screen is much like a single point phase detect AF system - the amount and direction of focusing needed to get it spot on are represented by how far out of alignment and which way out of alignment the two parts of the split image are. With not much practice, accurate and fast manual focusing us possible with a split image focus screen.
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