I had a discussion with Stephan Shulz, the project manager for the S2 a few days ago to get answers to some of the lingering questions raised after the specs were announced.I never understand why so many cameras come so close yet fail to reach 100%. Why is that? Does it save a huge amount on the mirror or prism size? I agree it's bizarre that a camera striving for such perfection, at a price that should ensure perfection, would do this.
Firstly, Leica measures the coverage of the viewfinder in percent of image area. Canon, for example, measures by linear coverage. The reality is that at 96%, the viewfinder is "cutting off" only about 0.2mm on each side. Measured linearly (same as Canon's method), the S2 viewfinder is 99%.
The reason for lack of 100% is that the sensor requires shimming and a little bit of movement in the calibration process druing assembly. In order for the viewfinder image to correspond to what the sensor records, there needs to be this 0.2mm spill to cover adjustmnets in assembly.
Be wary of any manufacturer stating 100%. They usually mean 99.x% on a linear basis and round up to the next whole integer. I'd be curious to see if anyone is willing to run a test on a camera with a "100% viewfinder" with some grid paper and share the results. I believe this is another case of Leica erring on the side of conservatism and it has nothing to do with physical size of the housing.
David