Are bigger pixels less noisy?

Started 4 months ago | Discussions thread
bobn2
bobn2 Forum Pro • Posts: 69,747
Re: Are bigger pixels less noisy?

J A C S wrote:

JahnG wrote:

Iliah Borg wrote:

JahnG wrote:

Camera manufactors like Canon, Sony, (Panasonic) have made ”low noise” cameras with less and bigger ”pixels”. Do the manufactors not know what they are doing? (Some people in DPR say that such cameras are manufactored because the customers expect that sensors with bigger ””pixels” would be less noisy?)

Suppose 1 large pixel collects the same light as four smaller pixels occupying the same space. Is it possible?

Sounds logical.

The combined light collecting area of a FF sensor might (roughly) be the same, regardless of if there are 20M bigger or 50M smaller pixels.

But how about the combined read noise of 50M pixels compared to the combined read noise of 20M pixels. One might thus think that the sensor having only 40% pixel count would have much less combined read noise? (Or shouldn't we sum read noise?).

The engineers may chime in why - but what I see is that smaller pixels have smaller read noise. In the end, read noise per unit area seems weakly dependent on pixel size.

I had a huge great argument here with Eric on this topic some years ago. Anyhow, this is what I was saying, reworded a bit, in deference to Eric.

When you're looking for dependencies, you end up making some assumptions, which are likely not realistic, but if the reality carries some correlation with the assumptions, maybe you've captured 'weak dependency'.

So, let's assume that all pixels are designed the same (of course, they aren't) and we produce small pixels just by scaling large pixels down proportionately. This means that every feature of the pixel gets smaller (and also, another discussion, that it's fill factor stays the same). The input referred read noise depends on the electronic noise from the pixel source follower and downstream and the conversion gain of the pixel. In turn the conversion gain is inversely proportional to the capacitance of the SF gate and floating diffusion. If these are scaled down the capacitance goes down, the CG goes up and the read noise is reduced.

Now, as said before, pixels aren't in reality designed like that, but we can see why there might be a correlation. For instance:

Companies that have multiple fab lines will tend to use the smaller process nodes for smaller pixel sensors, so the features in those pixel designs will be smaller.

If the sensor is being able to cope with a particular saturation exposure (e.g. to allow use with 100 ISO) then at that exposure the smaller pixel collects fewer photoelectrons, and can be designed with a lower saturation capacity. That means smaller capacitance in the SF gate/floating diffusion.

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