Marc Sabatella put me on to the link below:
Manual Focus with AF DSLRs
While the topic of the Web page is not directly related to viewfinder brightness, the authors make an interesting assertion. For the camera they tested, the viewfinder had the same brightness from for anything larger than f/4.
"...Even though the lens was stopped down a full three stops from f/1.4 to f/4.0, the screen brightness only changed slightly with a small visible difference only appearing after the change from f/2.8 to f/4.0. The background measurement changed from an average reading of 215 to 203 using Photoshop's picker. That works out to less than a third of a stop in brightness change when I compare it to a Stouffer 4110 transmissive step wedge image take with the C7070. A 1/3 step in the Stouffer image at an image value of around 215 shows up as a numerical value change of more than 15. Since most DSLRs meter from the focus screen, it is easy to see how using a fast lens can give exposure problems as well..."
Apparently, some focus screens "eat" light.
Steve