D600 High ISO in DX

Started Nov 23, 2012 | Questions thread
noirdesir Forum Pro • Posts: 13,622
Re: pixel pitch and SNR
Leo360 wrote:

noirdesir wrote:

In an ideal world, read noise is proportional to pixel size.

Why is that? I think that in ideal world read-noise should be independent of pixel size and should only depend on the temperature (even ideal world has thermal noise).

So, let's take a 2x2 μm pixel with a read noise of 6 e- (ie, a standard deviation of 6 e-). Now compare that to a 2x2 pixel array of 1x1 μm pixels, where each pixel has a read noise of 1.5 e-.

I don't think so. What makes you think that read-noise std.dev. scales quadratic with pixel pitch. Please, provide some rational for this behavior.

This is based on explanations by bobn, you'll have to ask him for details. But I have seen anybody showing that this premise as first order assumption was incorrect. And it does not sound unreasonable to me that read noise could be partly based on a phenomena that is proportional to the size of the pixel.

Simple statistics tell us that the read noise component for a combination of these four smaller pixels is:

rn(com) = square root of (1.5^2 + 1.5^2 + 1.5^2 + 1.5^2) = 3 e-

The formula above is based on a questionable assumption.

Following your line of thoughts I will consider 2x2 um pixel against a group of 100 of 0.2x0.2 um pixels with the read-noise sigma= 0.06. This follows from your own read-noise scaling rule! Now based on your summation formula I will conclude that

rn = 10x0.06 = 0.6 which is 10 times smaller that original 2x2 pixel.

After signal aggregation from all 100 micro-pixels the 2x2 pixel SNR increased 10 times. Seems too good to be true or is it?

Nope, only the noise contribution from read noise decreases that much, there is also photon shot noise which for most situations will swamp the read noise, thus SNR will not increase nearly as much as only looking at the read noise component will tell you.

And I said that this is an idealised assumption that only gives a general idea of one of the phenomena involved.

Bottom line, sensor noise performance is messy and contains lots of proprietary stuff in processing chain that we are not privy to.

That is why I added several real world examples where smaller pixels result in either equal or lower read noise contribution to total noise. My aim was not to postulate that read noise only depends on pixel area size but to refute those who claim that smaller pixels will result in more read noise in the final image.

So far from what I saw on Bill Claff's charts is that D600(DX mode) dynamic range outperforms D7000 at all ISOs. I have no reason to think that with D5200 it will be any different.

Bill Claff's dynamic range definition (PDR) is based on a noticeably higher minimum SNR than the engineering definition of DR. As a result, photon shot noise (and thus QE) have much higher influence than read noise for his results. And the D600 has a higher QE than the D7000. Moreover, Bill's PDR chart is also plotted over manufacturer ISO not an ISO normalised to saturation which adds another usually small source of bias for some cameras.

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