![]() Hence the gain for ISO 100 is half that for ISO200 etc. I guess this is what is happening with your different "ISO" settings. ![]() On the other hand you might sacrifice precision to avoid saturation of bright sources by reducing $g$. In most astronomical CCDs I have used, the gain is set so that the readnoise is properly sampled. Ī plot of variance in $n$ vs $n$ will thus have a gradient of $g$. The variance of the detected photons will be $n/g$ (just Poisson noise, ignoring the readout noise), but the variance on $n$ will be $ng$. Thus if you record $n$ adus, this corresponds to $n/g$ detected photons. In a CCD camera you have a parameter called the "gain" $g$, which can be defined as the number of analogue to digital units produced by each electron read out (which is the number of detected photons). I do not know what your DN units are but I doubt they are detected photons. The unit, DN, is the Digital Number readout of intensity which goes from 0 to 1024 in this camera. Here's the same effect in a paper ( Pagnutti et al 2017) on the Raspberry Pi Camera:Īs you can see, at each ISO tested, there was a linear relationship between mean and variance, but the slope differs based on ISO. One of them, read noise, is actually a constant term for a given camera and gain/ISO setting. ![]() Dark noise and read noise are not the major effects here. We have to account for additional sources of electronic noise, such as read noise and dark current: SNR S/SQRT (S + DC + RN2) Dark current and read noise are different in nature. The images are at a significantly long shutter speed with light present.Why is this? If photon shot noise is poisson distributed, why isn't the variance equal to the mean? I suspect the answer has something to do with the area of my pixels - am I somehow effectively taking multiple samples from the poisson distribution and averaging/adding them? Additional info in response to questions The Wikipedia article on shot noise in optics reflects this relationship, using " $\propto$" rather than " $=$" but does not explain why. I've taken some data in uniform lighting conditions and measured the variation of values of a particular pixel over time.īased on the poisson nature of shot noise, I would expect the variance in the pixel values to be equal to the mean pixel value, however I'm seeing that the noise in the pixel values is proportional to the mean pixel value but significantly smaller than it. I understand that on top of the read noise and dark noise inherent to the sensor, the data is also subject to photon shot noise. I'm currently working to characterize the noise in a CMOS camera.
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