Bart_van_der_Wolf
pro member
Hi, Bart,
Well, I'm working my way thorough all this. I am having a little problem with your chart. I have assumed that the six "grayscale" patches on the mini CC that you mention are intended to be the patches often called 24 through 19 (from darkest to lightest).
That's correct. 1=19 (White), 2=20, 3=21, 4=22, 5=23 and 6=24 (Black)
But I am surprised that the noise values for "patch 6" on your table are all smaller than for "patch 5".
Normally, we would expect the noise values (for any given exposure setup and ISO setting) to rise monotonically with luminance, owing to the shot noise varying as sqrt(N).
That's correct at the Raw level, but we're looking at demosaiced and gamma/tonemap adjusted images here. I wanted to see what we end up with after the data crunching, as that is what we'll base our visual judgement on. Maybe C1 has a built in shadow noise reduction, we don't know for certain. I switched off all noise reduction and sharpening that I could.
All I can think of is that your exposure was so high that the lightest patch always saturated (which of course would cause a decrease in digital noise), but that doesn't seem likely given the relationship between the various entries in the "ISO 1600" group.
The final target values for the white patch 1/19 were R=G=B=231, and for the Black patch 6/24 R=G=B=46, so there was no clipping involved. The black isn't very black, because it is semi-glossy/matte. I didn't want to stretch the contrast as one might do if there are no real blacks in an image, I only tweaked the exposure slider for identical output RGB values.
What am I missing?
Nothing, but I can't explain it either. It's perhaps best to do one's own test, even if it is only with a slightly defocused gray card instead of a step wedge scale.
One more question while I've got you up. Is the "native ISO" the one at which the photodetectors are saturated from an electronic standpoint (the so-called "well full" level) at the same photometric exposure at which the DN tops out?
That would depend on one's definition of native ISO. My measurements at the Raw non-demosaiced level show a maximum Dynamic range (engineering definition) and minimum gain at ISO 100 for the 1Ds3 I have (ISO 80 for my 1Ds2). Others have reported similar findings for other Canon models. ISO 100 is also the lowest standard ISO the camera's offer. Nikons seem to be closer to ISO 200 (although I don't know if that depends on the source of the chips, Sony or another manufacturer), but I haven't measured that myself. That would indeed require an exposure at the clipping level, and a noise floor, and must be measured at the Raw data level. ISO 'L' is ineffective in the 1Ds3 as far as Dynamic range is concerned (It's just ISO 100 gain pulled 1 stop in the camera).
Cheers,
Bart