The argument above provides a method to compare cameras with different resolutions. A high-resolution camera can still be turned into a low-resolution camera by averaging its pixels.
order to compare all cameras and not just pairs, a reference resolution can be
chosen and equivalent SNRs at this resolution are computed. Let Nref
denote this reference resolution. Consider a camera with N Mpix. To
obtain an image with Nref pixels, N / Nref
pixels need to be averaged out to produce a single pixel in the output image,
so the noise decreases by a factor of .
We then deduce that SNRref at resolution Nref
can be obtained from the nominal SNR (in dB) using the formula
Notice that if the reference resolution Nref is very large, or if the initial sensor has a low resolution, the normalized SNR can actually be lower than the nominal SNR.