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Effects of measured versus set ISO

mabuge

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mabuge Registered since 18th Mar 2012
Thu 12-Apr-12 12:49 PM | edited Thu 12-Apr-12 07:03 PM by mabuge

DxO has some nice graphs related to the D800 and other camera's, with one typical point that strikes the eye: measured ISO is different from the set ISO... for all camera's. The same goes probably for aperture and speed, but that's not the subject here. Accordingly, my questions are more of a general nature. I used DxO's D800 figures in the graphs below.

http://www.dxomark.com/index.php/Publications/DxOMark-Reviews/Nikon-D800-Review/Sensor-performance

The top graph shows that the difference between measured ISO and set ISO varies from 63% to 75%, with one exception: the 50 and 100 setting have the same measured ISO ! (The graph has a manipulated logarithmic scale to make it look better, that's all.)

Let's start with the latter. What are the effects of the 100/50 settings? From my limited knowledge, there is no effect on the resulting speed and picture, except for the noise level. Measured ISO is 74 for both, that is to say, for the D800 tested by DxO. Because noise level coming forth from amplification is higher with higher ISO, would the 50 ISO setting be the better one? If so, why did Nikon use 100 as the base ISO level iso 50? There must be a reason that I fail to see.

Unfortunately DxO did not take 50 ISO into account in their SNR 18% graph (and by the way, in all but the first graph, the vertical axis crosses between the tick marks, a bit annoying). I'm referring to the detailed measurements when you click on the link called "Nikon D800 vs Nikon D4".

About the second graph below, just another way of looking at the same data. The set ISO settings ratio is exactly 2 (I added the blueish horizontal line as a reference). On the logarithmic scale you see the red (measured ISO) and the orange (set ISO) setting incline about the same way. However, with an exception made for ISO 100 and 50, where the ratio is 1, the other ratio's vary actually from 1.85 to 2.06 (about 11%). Look for example to the ratio drop from 800 to 1600 (measured 594 and 1099). What could be the effect of that, except for that the camera is a tiny bit less faster than you expected ?

When looking at the measured data, except for the first one, the actual ISO is more conservative, which should have a positive effect on noise. The only negative point is that with the highest setting, 25600, you only get 16117, a 37% difference. But does that really matter? You don't choose this camera for the ultimate low light situation.

I don't want to be negative about the deviations, nor Nikon, as I said, all camera's appear to have them. It's just curiosity that drives me here.

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