Daaaaaaaamn! Check out the D5200 sensor!

Started Jan 24, 2013 | Discussions thread
bobn2
bobn2 Forum Pro • Posts: 56,024
Re: Aptina?

Jack Hogan wrote:

dethis2 wrote: What this assumes is that the grey scale is linear with respect to photoelectrons collected. If at the top of that things are compressed, that is it takes more photons to produce the same grey level change (because the signal is in the region of the shoulder of the output transistor characteristic) then the apparent number of photoelectrons collected will be smaller than the actuals - which will cause the effect you observe.

How can we detect this non linearity ?

One way I am thinking of, is inspecting the histograms for the distribution of a well lit homogeneous area. Like the white patch of the Greyscale#14 at ephotozine's samples. It is about one stop lower than saturation.

Except that it's not that non-linearity we are after, I think. As you can see from the relative graph, the ISO100 curve is as well behaved as the others, so there is nothing there suggesting that as the exposure (time) is reduced the mean output raw values behave non-linearly, especially if we are observing the shot noise only portion in the middle of the curve (say .5-5% of full scale).

Well, the curve fit extracts the well behavedness of that curve, and gets the anomalous results.

I believe the answer lies in a different unity gain being applied at ISO100 vs the rest , resulting in a different G in Bob's formula: take a look at the 'relative gain' in my table to see what I mean. Viewed from the point of view of the raw data, the sensitivity of the sensor has changed differently than expected at ISO100.

I'm not sure what you mean by 'unity gain' - the gain is the gain. The only reason that I can think for the anomaly is that what should be linear gain (a single multiplier term G) has become a non linear gain ( f(G)) introducing effectively another term in the equation. This is what would be expected given the electronics, and behaviour of a MOSFET near saturation, and also seems to fit what is observed.

What I do not understand is why this isn't being picked up in DxO's supposedly accurate Sensitivity (Ssat) measurements. Steen Bay has reasonably suggested that Ssat is not really 69. Iliah also seems to think so. If it walks like a duck, quacks like a duck...?

Remember that the non-linearity I am talking about would only manifest itself at the top end of the scale, near to saturation level, you wouldn't see it anywhere else. Remember also that DxOmark's curves are themselves fitted to their measured data, so if the 100% saturation was off it might be flattened.

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Bob

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