Moon Maid's sensor comparison (PART 2)

Started Mar 1, 2012 | Discussions thread
bobn2
bobn2 Forum Pro • Posts: 58,572
Re: D800 read noise climbing more slowly than ISO

Marius D wrote:

Marianne Oelund wrote:

I haven't included this effect in the posted curves because I need more data before I'll accept it, but it does appear from the samples I have, that the D800 read noise (in ADU) is climbing more slowly than proportionality to ISO. On the surface, one wouldn't expect this, since ISO sensitivities above ISO 1000 (if it follows the D7000 example) should be only by numerical scaling.

I can see two possible explanations: 1) Some gain sharing at high ISO, such that some of the boost is still provided by the analog electronics; 2) A slight amount of NR is being applied to the RAW data.

There is another possible explanation: 3) reducing ADC reference voltage (V.ref) to match the input analog domain as ISO raises. This should appear like a numerical scaling, except for reduced quantization errors. My guess is they are using both analog amplification and V.ref scaling. From around 1000 ISO up to 6400 there is no analog amplification, only V.ref scaling with a small but measurable contribution in reducing read noise due to the optimized quantization and other factors like better reset voltage reading or optical-black photo-detectors reading. Reset voltage and optical-black are both very low, reducing ADC's V.ref make sense for a precise digital conversion of these values.

There is another possibility. I suspect that Nikon is actually adding a tiny bit of pseudo-random noise to provide a little dither right at the noise floor, my guess is to hide some of the very low level artifacts of some of the digital signal processing that they do (remember the EXMOR sensors are direct digital out, so there is little scope for analog signal processing apart from manipulating the configuration options that Sony has given them). If you look at the D7000, the high ISO read noise is a bit higher than other cameras using similar sensors. Possibly in the D800 Nikon has graded the added dither a little.
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Bob

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