>>According to DXOMark "low-light ISO is the highest >ISO >>setting for a camera that allows it to achieve an SNR of >30dB >>while keeping a good dynamic range of 9 EVs and a color >depth >>of 18bits." >> >>Yes? >Ok. What, exactly does that mean to you as a photog?
Fundamentally, it means that at ISOs above those listed, one or more of the image components listed (EV, color depth or SNR) will begin to suffer perceptibly. According to DXO's comments, a 25% change in the ISO number is equivalent to about 1/3 EV. So based on that comment, it would appear that the gap between the D3s and the D4 is statistically insignificant as far as the picture is concerned.
Be that as it may, one would certainly expect the ISO performance of a generation newer sensor such as in the D4 vs the D3s, to exhibit an IMPROVEMENT in performance, even if very slight. However it does not. I am simply wondering why not. Gains were made in the other two primary categories of performance denoted.
So if I may take the liberty, the thrust of your comments regarding this discussion seem to be:
1. For the same generation of sensor, pixel density has no bearing on ISO/noise performance
2. Relying on DX to replace a 1.4x tele extender is folly. Particularly in situations where noise is going to be an overriding factor.
3. Noise performance can only be bettered by increasing the light gathering power of the front element. Corollary here is that my 300/2.8 should do SIGNIFICANTLY better than my 70-200/2.8 because the aperture isn't a factor, merely the size of the front element. And in this case, it's more than twice the area.
So given all this, it seems that the answer to my low-light issue is to buy a lens with the largest front element I can find, and attach it to a D3s or D4.