# Getting down to the nitty-gritty about noise and it's effect on IQ Locked

Started Apr 15, 2010 | Discussions thread
Re: Total noise power

Steen Bay wrote:

I'd say yes, a higher resolution image has more noise.

IMHO, you are conflating a number of issues here. If the noise power spectrum reaches out to higher frequencies in a higher resolution image that does not necessarily mean that a given pixel in a higher MP Image has more noise. Consider this: If all other variables are the same then lets assume that a pixel in the lower MP image has a mean signal of N. Then the "usual" notion of Poisson noise power would be N (SNR=sqrt(N)). However, if we assume a set of 2x2 pixels in place here and assume that the mean signal in the smaller pixel is now N/4, then the noise power is N/4 (SNR = sqrt (N/4). So the actual average noise value on the smaller pixel (higher MP image) is smaller than a bigger pixel in the lower MP image, but its SNR is poorer, of course.

So what do you mean then that the "higher resolution image has more noise?"

The 20mp image has the same noise at the shared frequencies as the 10mp image, plus the additional noise from its higher frequencies,

Using the numbers from above under the assumption of stationary white noise the power spectrum for higher MP has a flat value of N/4, where as the power spectrum of lower MP image has a flat value of N, then how do you claim that in the shared range of frequencies the noise is the "same", whatever that means, which is not entirely clear to me as I mentioned above.