more pixels are better!

Started Dec 14, 2008 | Discussions thread
ejmartin Veteran Member • Posts: 6,274
Re: Are those figures for "print" or "screen"? (nt)

ejmartin wrote:

Les Olson wrote:

I disagree. For any quantity the correct approach is to compare
first the actual value. Adjusted, normalised, or standardised values
can be compared IF they serve a purpose, and it does not matter
whether you are talking about "seasonally adjusted unemployment" or
normalised SNR. The trap, as in this case, is to use normalisations
that encode ideas about the right answer.

Except that comparing values of a quantity X that depends on another
quantity Y at two different values of Y is quite silly, unless one
knows (as is the case here) how to translate a measured value X(Y1)
into its value X(Y2) for the purpose of comparing to another
measurement of the X at Y2.

Furthermore, it is not as though the noise at a fixed scale cannot be
measured. It is merely that it is more convenient to measure the
noise at the pixel level and use well established properties of pixel
noise to infer its value at any other scale.

Just to amplify on this a bit, the noise power as a function of spatial scale (frequency in lines/picture height -- finer scales to the right, coarser scales to the left) looks like this for a 40D (red points) and 50D (blue points):

They are the same for all practical purposes at any given spatial scale. The std dev of noise per pixel, however, is proportional to the square root of the area under the curve, and so is higher for the 50D than the 40D, simply because the 50D has greater resolution (image data at higher spatial frequencies), not because it has more noise at any fixed spatial scale. Measurements of pixel level noise, that do not take into account the scale dependence of noise, are a naive and misleading presentation of noise characteristics.

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