I saw some surprizing results of comparisons between the Sony A900 and other cameras, like the A900's DR being a full stop greater than the D3X' and the 5DMkII's.
I set out to measure the noise and DR myself, and I soon ran into a problem: the measurement results were iconsistent, in some cases plainly contradicting each other; for example the noise measured on the raw channels separatedly is different between the channels.
It appeared that I needed better, i.e. for measurement more suitable raw files. Erik Kaffehr, a fellow poster here and a A900 owner volunteered to create the images; after some fine tuning, he delivered excellent bases for measurements.
(Btw, Erik's contribution was much greater than it is usual when I am asking someone and receiving specific raw files. In the span of several weeks he shot again and again, all together about 100 images. Those, who appretiate this report should say or think a big "thanks" to him.)
Then the surprize came: the measurement results were strange even with the best images. I had to dig deeper in the analysis (thank to Rawnalyze, this was not a problem). Some of the findings were astonishing.
I will create a much bigger essay with painfully pixel-peeping documentation of the background (I created hundreds of screen captures for this purpose), but that is a tiresome business, it will take some more time, and anyway many photographers are allergic to proofs, which are too factual; this is particularly true on these forums.
Here are the findings in condensed form:
1. Noise reduction Off, Low, Normal and High are identical from ISO 100 to 800, ON THE RAW DATA. They correspond something like NR Low @ ISO 1600.
In other words: all reviews used pre-NRed samples.
However, NR Off means almost Off with ISO 1600 (this explains the "sudden" increase of noisiness at ISO 1600).
2. The noise reduction affects mainly the red and the blue channels, much less the green. This is a very primitive noise reduction; it simply eliminates some pixel levels in the affected areas.
3. Due to the nature of this noise reduction, the very dark, noise reduced areas become darker. The consequence is, that not only the visual appearance but the noise measurement too indicates not only lower noise that it would be without NR, but that in deeper shadow, suggesting a greater dynamic range.
The magnitude of this shift is about 0.6-0.7 EV between the red/blue and the green channel. As the green channel itself is not virgin either, the shift must be even greater.
I estimate that in end effect the noise gets shifted and the DR "enhanced" by at least one full stop.
A side effect of the above is the non-linearity of the pixel values, and a WB shift in the low ranges: if the WB is correct in the "normal" ranges, it is certainly incorrect in the very dark range.
The nature of this noise reduction explaines the blotchiness as well. This will be appearant from the coming demonstrations.
Anyway, after dozens of hours squeezing these images I still can not put reliable numbers on the noise of the A900. However, I can say, that the noise is considerable higher and the DR is considerable lower than some reviews have suggested.