According to the company, the Canon 1D Mark III's pentaprism viewfinder has a coverage of approximately 100% (which agrees fairly well with our test results), a magnification of 0.76x (-1 diopter with 50mm lens at infinity), an eyepoint of 20mm, and offers a diopter adjustment range of -3 to +1. The viewfinder hasn't changed that much from the Mark II N, except that a few important icons have been added. There's a new metering mode indicator, plus RAW mode and Battery level indicators. Also, the new Laser-matte screen does seem to help differentiate between in-focus and out-of-focus objects. The screen is interchangeable, with 11-types being offered. A built-in eyepiece shutter is also provided, for when you aren't using the viewfinder.
See below for callouts of the information available (diagram courtesy of Canon USA):
Viewfinder Test Results
Coverage Excellent accuracy from the optical viewfinder and "Live View" LCD monitor.
100mm, "Live View" LCD
The Canon EOS-1D Mark III's optical viewfinder showed about 99% frame accuracy,
while the "Live View" mode of the LCD monitor showed just under