More megapixels, better photos: Fact or fiction?

Started Feb 7, 2008 | Discussions thread
ejmartin Veteran Member • Posts: 6,274
Re: So many misinformed replies

Eric Fossum wrote:

ejmartin wrote:

Why do you think read noise goes down with pixel size in the examples
I listed? It's because whatever is counting the electrons has a
smaller sample to deal with, and thus makes a smaller counting error.

This is not read noise. Read noise is signal independent and is the
noise contributed by all the elements in the signal chain. If the
same transistor and signal chain is used by two different pixels, one
large, one small, then the read noise will be the same between the
two pixels.

Quite right, some fuzzy logic there indeed.

If read noise is decreasing, it is probably because the pixel is
being reengineered to reduce noise, although changed operating
voltages can also affect the noise.

You are getting at what I meant to say. As pixel size decreases, the maximum count of electrons that need to be counted is reduced since the well depth is lower. The electronics involved is beyond my expertise, however I was reasoning that if, in a fixed period of time, the readout circuit has to be able to count Nmax electrons on a big pixel, but only needs to be able to count say Nmax/2 electrons from a smaller pixel, that the engineering constraints on the big pixel readout are more demanding than the small pixel readout.

But I don't know the details of the circuit designs. I do know what I and others have measured, and the result seems to be that read noise decreases with pixel size; even read noise per unit area slightly decreases with pixel size. In current camera models, that is, and at low to moderate ISO.

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