ejmartin
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 p.3 #2 · Dynamic Range Measure | |
Alan321 wrote:
ejmartin, please forgive me if I'm being a bit dense (it happens more frequently these days  ) but how is measuring DR on a per image area basis any different from a per pixel basis ? Wouldn't you be scaling both noise and signal by the same amount and hence getting the same DR result ? Or am I talking about the wrong quantity, being a signal to noise ratio instead of an absolute electron count ?
- Alan
You are right that DR and S/N ratio are two different things. DR is the ratio of maximum signal to minimum noise, which is the noise with zero signal; S/N ratio is the ratio of the signal and the noise attendant to that signal. Since the noise increases with signal, these are two different things.
The answer to how things scale: Signal (number of photoelectrons) increases proportional to the area, and thus to the number of pixels in a patch of sensor. The noise in each pixel is independent from other pixels to a very good approximation, and so combines as the square root of the sum of the squares of the noises of individual pixels. If each pixel has the same noise (true for a uniformly illuminated patch), the noise will increase as the square root of the number of pixels combined. So both the DR and the S/N ratio go up as the square root of the number of combined pixels. If comparing sensors of different sized pixels, the per area DR is the per pixel DR scaled by the corresponding linear pixel dimension.
Alan321 wrote:
I'm also a bit surprised and disappointed to see the 1Ds3 result lower than the 1Ds2, 1D3 and 40D results. Can anyone (Stan ?) confirm what these numbers mean anything in practice - i.e. does the 1Ds3 seem to have less DR than the 1D3 or 1Ds2 ? If it did then I'd expect it to handle high iso less cleanly. If it seems to be cleaner or at least as clean as the 1D3 then it would seem there is a discrepency in the test results (despite being re-tested several times). Maybe it is mathematically noisier afterall but the noise is visibly finer and less intrusive because there are more pixels per image ?
- Alan
The answer to this follows from the previous reasoning. The 1Ds3 pixels are smaller; if you look on a per area basis, the 1D3 and the 1Ds3 have the same DR.
And this is the proper comparison. When you look at a print, you take it in as a whole, and you look at the objects in it which are a fixed percentage of the frame regardless of how many pixels they contain, so the appropriate figure of merit is to refer DR, noise, etc, to a percentage of the frame, or if considering a given sensor format, to a per area basis.
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and tell us what it means ?