I see what you are saying, maybe I'm using the wrong term. Let me try to explain what I'm attempting to figure out. When I look through the VF of my 5D, the focusing screen is fairly large, but it in no way fills my entire field of view (i.e. I can still see the black frame around the focusing screen and the eyepiece without moving my eye). With a very high quality microscope, telescope or binoculars when you put your eye up to the eyepiece it seems to fill your entire field of view (very little preceived black area around the projected image).
Now, this morning I measured the actual opening through which you look on my 5D, and the opening is approximately 15mm x 10mm. On an XSi (had one sitting on my desk) the viewfinder opening is much smaller, about 11mm x 8mm (and has rounded corners).
One other observation I had is that if you place the viewfinder to your eye and focus on the center AF point and slowly move the camera away, the AF point appears the same size, regardless of the distance between your eye and the VF. So, in essence I guess what I'm asking would be the product of eye relief and magnification. I have no experience with MF cameras, does anyone with experience with MF cameras know how format effects how much of your perceived field of view is filled with the viewfinder?
Does anyone know what is preventing camera manufacturers from producing VF with magnification as large as the manual focus cameras of yesteryear (other than probably R&D development costs and non-existent ROI)? Example:
Pentax MX - 0.97x / 95 % / 1.0x crop = 0.92 (best 35mm format MF viewfinder I know of)
Nikkormat EL2/FT3 - 0.90x / 92% / 1.0x crop = 0.83 (entry-level Nikon circa 1977)
Canon AE-1 - 0.86x / 96% / 1.0x crop = 0.83
Olympus OM-4T - 0.84x / 97% / 1.0x crop = 0.81
Nikon F/F2/F3 - 0.80x / 100% / 1.0x crop = 0.80
Also, it appears that a larger eye relief could be acheived by using slightly larger lenses in the VF and a larger opening (say, a 18mm x 12mm or 21mm x 14mm eyepiece). Anyone know the opening size for the higher eye relief cameras (and/or higher magnification)?
Canon 1D X - 0.76x / 100% / 1.0x crop = 0.76 (best 35mm format AF viewfinder ever)