Originally posted by dosdan If ISO affected the exposure, then these two settings would produce "equivalent exposures". But they don't. Setting #1 captures 4x the photons captured by #2. Since SNR due to shot noise is the sq-root of the number of photons, #1 will have 2x better Shot Noise SNR than #2. For example, if a pixel exposed using #1 captures 40,000e- (photo-electrons), it has a photonic SNR of 200:1. While in #2, there would be 10,000e- captured, with a SNR of 100:1.
This is why I'm hoping that future Pentax DSLR cameras will give us a little extra at the LOW end of the ISO scale. Forget shooting into the half-million; I'll be quite happy for my sensor to top off at the 51,200 I currently enjoy from my K-5 if they can push the low end down to fifty or even 25.
Give me that and a centre split-prism and I might even think about upgrading (especially if they un-cripple the K mount for the old M lenses).