Help understand

Started Oct 20, 2009 | Discussions thread
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rhlpetrus Forum Pro • Posts: 25,993
Help understand

I read now and then that "native" ISO is so and so, usually meaning some highest value which sensor is still producing decent results, above that it is just digital multiplication of results.

But exactly what does that mean? Assume, like D3/D700, that base ISO is 200, so that's where the sensitivity os sensor will provide the lowest SNR and highest DR, in principle.

Now double the ISO setting to 400. In principle again there's half the light being recorded for same scene, so that the sensels are not saturating if before they were just getting filled up with photons.

To me it already means there goes 1 stop of DR, or no?

102K ISO means 9 stops lower light than ISO 200, thus how many stops of DR is the sensor recording? If originally it was, say 12 stops, now it's 3? Given that the lowest 3 stops sually mean mostly nothing, just shades of noise, what's going on?

I checked the DxO Mark graph for Dr on the D3:

http://www.dxomark.com/index.php/eng/Image-Quality-Database/Nikon/D3

It seems that for the first 3 stops (actually 161, 326 and 625 as they measured it), the reduction is not as I would expect, it's superlinear (above the -1stop Dr/1stop ISO), then it gets linear, as expected, -1 stop DR/1 stop ISO.

So, from this data, the D3 would be trying to have better than expected DR up to 800ISO setting, then above that it's basically surviving in terms of DR, SNR is similar.

We accept some loss for higher ISO, depending on application, but we know the image IQ is degrading from the start.

It's interesting that the D3x starts higher but looses ground faster right at the beginning as ISO goes up, so that at about 1600 ISO they are tied re DR (at same print size, snd go together). How's that? Why D3 holds DR longer, how's that engineered?

Then again, what's "native ISO"?

Now the D3s: 102K ISO means 9 stops lower light than ISO 200, thus how many stops of DR is the sensor recording? If originally it was, say 12 stops, now it's 3, maybe 4 given the DxO Mark graph for D3? Given that the lowest 3 stops usally mean mostly nothing, just shades of noise, what's going on? Has Nikon cooked the first few ISO steps so that DR goes down a bit slower, or has base ISO DR gone up to compensate?

Why didn't Nikon use same trick for D3x, to keep its DR higher as ISO goes up?
--
Renato.
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