# f-number equivalent between m43 and FF

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

Great Bustard wrote:

dwalby wrote:

OK, let me reword my statement a bit. A pixel contains an analog signal value prior to A/D conversion. Every analog signal has some sort of noise associated with it, and the A/D conversion itself introduces read noise. So each pixel, when sampled, has a signal level, and some uncertainty due to the noise.

The amount of light falling on the pixel during the exposure is subject to uncertainty, due to the nature of light itself. This is known as photon noise. Then the sensor adds additional noise, which is called read noise.

Yes, I know that, it just wasn't important in the context of the original discussion regarding SNR being attributable to a single pixel.  I was only discussing the basic concept of noise being present, not the nature of the noise itself.

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.  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.

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.

Regardless, it is *crucial* to understand that noise comes from the light itself (photon noise), and that this noise is an inherent property of the light, having nothing, whatsoever, to do with technology. The sensor and supporting hardware then adds in additional noise (read noise).

again, already understood.  I believe the noise distribution is Poisson when a very small number of photons are counted, and becomes Gaussian when a larger number of photons are counted.  And as such, if the signal voltage created by the photons is S, the noise voltage (standard deviation) is the square root of S.

And, in the example above regarding averaging a patch of pixels, the average sum of N signal voltage values is NS and the standard deviation increases as the square root of N.

Except for the portions of the photo where the light is very low, the photon noise is the dominant source of noise. Thus, for deep shadows and/or very high ISO photography, the read noise becomes an important factor, but until that point, it is the photon noise that is dominant.

agreed, never challenged this.

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.

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.

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