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# The Joy of Pixel Density

Started Jul 13, 2008 | Discussions thread
Re: Shot noise

Tristan Cope wrote:

aclo wrote:

Tristan Cope wrote:

If the photon count
for each individual pixel gets very low, then then signal to noise
ration dramatically falls.

Let say, for example that we have a 40MP 1/1.8 sensor, and that in
certain light conditions the average photon count per pixel is aound
1. Because of the effect of shot noise the signal will be
indistinguishable from the noise. Binning pixels won't help.

The last statement isn't true. Can you explain the thinking that
leads to it?

Yes. Lets take the extreme case and say that one photon is falling on
each sensor element. Signal = 1, noise = sqrt 1 = 1. Signal and noise
are equal (essentially photon flux per sensor element is entirely
random regardless of the signal). This is a quantum effect.

That the signal and noise have the same magnitude is irrelevant, as I explained before (and others do later). And it also happens with classical objects being randomly deposited onto a surface (such as epitaxial growth), so it's not quantum (except inasmuch as the photons being discrete is a "quantum" effect). Anyway this is off topic.

OK think of this: imagine a photosite split in 10x10 smaller squares. Suppose I "detect" photons separately in each, and there is no read noise. Do you not see that shot noise will be unaffected by whether I a) write out all 100 values, then add them, or b) I have a computer (or analog counter) do it? (think about this a bit before dismissing it). [and the photons aren't coherent so we really can argue like above].

I hope the idea gets across with my example.

"Binning can be done in firmware after signal readout and
amplification, but this wouldn't reduce shot noise. To bin sensor
elements together in a way that reduced shot noise would require this
to be done in hardware at sensor level before readout. 2x2 (or 4x4)
blocks blocks of immediately adjacent sensor elements would have to
be combined before readout so they were treated as a single large
sensor element (i.e. a bigger "pixel"). This can't be done with CFA
sensors due to the CFA array. You would be binning pixels with
different colour information.

Yes, I agree that the presence of the CFA complicates things and makes all the discussions here not directly applicable. You can bin after demosaicing, but the demosaicing process introduces long-range correlations in the noise, and this decreases the effectiveness of binning.

That's a different story from the noise and signal being equal, though.

Is there any CFA camera on the market currently that claims to use
hardware pixel binning?

I don't think so, and it wouldn't make much sense, as you say. Even worse, just binning 2x2 and so on would cause aliasing, so things are even worse in practice.

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