D600 High ISO in DX

Started Nov 23, 2012 | Questions thread
bobn2 Forum Pro • Posts: 62,017
Re: Clarkvision.com analysis

Leo360 wrote:

bobn2 wrote:

Leo360 wrote:

I cannot speak for Roger but I am NOT comparing across pixel sampling frequencies.

Yes you are, you might not think so, but by seeking to compare DR or SNT 'at the pixel level' that is exactly what you are doing.Pixel size determines sampling frequency which determines recorded bandwidth but not observed bandwidth.

No, I defer SNR and DR comparisons to after the re-sampling and do not compare them for naked pixels. And down-sampling done properly tends to increase the SNR by rejecting higher frequency noise. See below.

On the contrary, I am re-sampling the higher freq. image (think down-sampling) to a common sampling rate and then comparing at the same reference frequency. And down-sampling (when performed properly) tends to improve SNR.

Down sampling is unnecessary, all that is necessary is to observe the images the same size. For instance an A3 print is 18MP on a Canon 300 ppi printer. On an Epson 360 ppi printer it is 25MP. So if I took, for instance, a 1DX image I could print at A3 on a Canon without significantly resampling. I could do the same with a D600 on an Epson. If I compared the prints the noise would be substantially the same, without any resampling having happened, simply because it is the viewing size and the acuity of the viewers eye that determines the viewing bandwidth, so long as the output device pixels are below the limit of acuity.

I want to keep viewing acuity out of it because it is yet another low-pass filter and the problem is already complicated enough.

I want to compare signal and noise per sample (trying to avoid word pixel when sampling on both cameras is normalized to the same bandwidth (the same sampling rate). The sensor is the same size also. IIRC, this is what you proposed yourself in one of your earlier replies in this thread. If I know signal and noise properties of samples at a different sampling rate (different pixel pitch) AND I know the frequency response of the down-sampler I can calculate effective SNR and DR after re-sampling to a low common rate. This is standard technique in Multirate Signal Processing. Doing so establishes the same effective bandwidth for both images and allows to compare effective SNR per sample as in apples-to-apples. This method allows for an automatic ala DxOmark comparisons without bringing a human eye into the picture.


I think that you are getting so far away from what you originally said now, that we are discussing on shifting sand. So long as you don't want to make 'per pixel' comparisons, I won't argue with that. However, trying to take the 'human eye' out of photography is rather futile (what would the point be?). What we are trying to do when we decide which objective quantitative measurements to make, is to choose those which allow us fairly readily to make some predictions about what a human eye would see. In a real viewing situation the bandwidth is, or should be, limited by the acuity of the viewers eye. How we choose to simulate that limited bandwidth when we make quantitative comparisons is a methodological issue. For instance, DxO does not resample to obtain their normalised results, they make a calculation based on a normalised output resolution.

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