From DXOMark: More pixels offsets noise3

Started May 30, 2009 | Discussions thread
ejmartin Veteran Member • Posts: 6,274
Re: Terms..

Steen Bay wrote:

The terms and language used in these discussions can sometimes be
pretty confusing, at least to me. For example I managed for a long
time to misunderstand what you and others meant by the term 'image
level'. When I was told that at 'image level' the noise was largely
the same, regardless of the pixel count (same sensor-size), then I
took it to mean that the total noise would be the same, when
comparing images at the same output size (not 100% view), like in big
prints.

I try not to use the term "image level" because it indeed has no technical definition that I am aware of, and different people mean different things in their usage of it. I always try to use terms such as "at a fixed scale" or "at a fixed spatial frequency" which are quantitatively defined.

Later Bob (bobn2, who unfortunately has been permanently banned
now..) told me that comparing at 'image level' was comparing images
at the same level of detail, meaning that the image with more MP's
would have to be downsized/blurred (and thereby lose most of its
extra detail) for the noise to be the same, again meaning that there
wasn't really any 'free lunch', as i thought at first.

Well, there you have an example of someone giving their definition of the term. I imagine JPS means something slightly different.

If we talk about noise power as a function of spatial frequency, no resampling need be done; just compare the noise power spectra of two images at a common spatial frequency. I showed those to be the same for the 40D and 50D. The 50D will have more noise at its Nyquist frequency than the 40D has at its Nyquist frequency, but that's comparing apples and oranges. The 50D also has higher pixel level standard deviation, since the standard deviation is the area under the power spectrum curve, and the 50D curve extends further due to its higher Nyquist frequency. Visually, the 50D will have a bit more noise as a consequence if its images are printed at the same output size as the 40D; there is after all overall more noise power in the image. It is all at high frequencies (higher than the 40D can produce), so it comes together with the greater detail.

For me, the extra resolution is about choices. If you want the extra resolution and the high frequency noise is not objectionable, fine. If you want most of the resolution and to get rid of some of the high frequency noise, use one of the better noise mitigation algorithms, which try to preserve image features while scrubbing the noise. The crudest thing you can do is apply a low pass filter (which is what downsampling does), which gets rid of all noise and all detail beyond the filter's cutoff frequency.

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tko
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