# f-number equivalent between m43 and FF

Started Mar 25, 2014 | Discussions thread
Re: Still incorrect.
2

dwalby wrote:

Bob is saying that a single pixel does not have noise, because he is computing the noise by taking the average value of a patch of pixels that should have uniform values and computing the standard deviation.

Well, Bob could have said that, but he seemed happier with one-sentence answers with no details.

Bob is rude enough to make the assumption in the first place that people who make claims will want back them up.

Computing the stdev of multiple pixels does not invalidate the assumption that an individual pixel can have an SNR value associated with it, it just changes the SNR computation.

How do you compute the SNR of a single observation of a single pixel?

However, we could compute the noise of a single pixel by instead taking several identical exposures, recording the values of a pixel, computing the mean, and then the standard deviation.

exactly, which is what I said earlier about having a 1 pixel sensor, then exposing it multiple times at the same exposure setting and seeing how the pixel value varied with each exposure.

Bob's statement explicitely excluded that, because it said that a single pixel in a single photo has no SNR. Saying there is variability over multiple observations in time is irrelevant with respect to a photo.

Either way, it is not per-pixel noise that matters, but image noise, and this is a function of the total amount of light falling on the sensor (and sensor efficiency), not the amount of light falling on a single pixel.

Bob stated that a single pixel doesn't have an SNR, I disagreed with that claim. Everything you said above is true, and I agree with, it just wasn't part of the original claim I disputed.

Wrongly.

I'm not disputing whether or not per-pixel SNR is the best metric for evaluation of a sensor's performance, I was simply stating that it is possible to measure and compare the SNR of a single pixel on any given sensor. Perhaps if Bob had taken the time to explain his own comments a little more thoroughly you wouldn't have had to do it for him.

You didn't do that 'for' Bob, you did that 'for' yourself, and still ended up being wrong. As Bob said, there is no SNR for a single pixel in a single photo (and when talking about what one photo looks like, considering what the next one might look like is a bit of a nonsense).

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Bob

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