5D artifacts

[John_Sheehy]

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Hi, John,

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Most converters don't treat noise symmetrically. They just call the center of the bell-curve of noise (from the black pixels) black, or zero, so half of the noise gets clipped to 0 (all the noise that would be negative).

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Exactly.

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I have to wonder how many different approaches were played with. I see multiple possibilities, some of them are:

1) treat pixels below the blackpoint as non-existent, as if they were defective (interpolate them from the "good" ones)

2) demosaic first, and then clip the black - this would cause some averaging which would push more of the resultant values above 0 before clipping - this would brighten the shadows a bit, and may require a subsequent transfer curve to darken

3) put the whole bell-curve of noise in the image, but compress it without affecting other tonal ranges relative to black - this would need to know white-balance parameters before-hand to do correctly

4) a combination of 2&3

5) just clip the average black value - this is what I think most are currently doing - and for some cameras, I think it is done wrong (the 10D has a green cast in deep shadows - possibly from a poorly chosen blackpoint)

[John_Richard]

"The implication of this is that if we did have the same sensors we have now, but had high-quality 16-bit digitization, you could set your camera to IS0 100 and have tremendous exposure latitude and/or DR." - but there are cameras with 16bit AD (like my Sinar 22mp) and this is simply not the reality - you cannot be sloppy with exposure and expect optimum image quality. Underexpose and you have to dig deeper into the shadows and image quality due to noise suffers - overexpose and you loose highlights.

[John_Sheehy]

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"The implication of this is that if we did have the same sensors we have now, but had high-quality 16-bit digitization, you could set your camera to IS0 100 and have tremendous exposure latitude and/or DR." - but there are cameras with 16bit AD (like my Sinar 22mp) and this is simply not the reality - you cannot be sloppy with exposure and expect optimum image quality. Underexpose and you have to dig deeper into the shadows and image quality due to noise suffers - overexpose and you loose highlights.

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I did not imply in any way that this is the optimal way to photograph. Your Sinar comments don't do anything to help your argument; we don't know how good the sensor is, how good the digitization is, etc. If it has multiple ISOs, it is possible that 16-bit at the higher ISOs can't do much because of noise.

What I *did* test, is that my most recent Canon DSLR has more detail and less posterization and noise at ISO 1600 than it does at ISO 100 exposed 4 stops lower (same exact sensor exposure). That means that 12-bit digitization is missing some shadow detail, and is posterizing the image at ISO 100. Do you understand this? There is more to be captured in the sensor than is being captured at ISO 100 with 12 bits.

[John_Richard]

It's not hard to grasp that 1600ISO images are not software adjusted versions of 100iso images - I'm sorry that I have not acknowledged this obvious point you made earlier but I am still not clear how this can only be interpreted at a 12bit problem. I do understand that signal amplification/gain is being used to raise the ISO (except in the 3200iso settting which is software) so is it not also likely this is an important factor? This is an interesting thread and I do respect your knowledge on this subject and also acknowledge my own limitations into the theoretical aspects of digital but I'm finding it very hard to tally my own practical experience of working with digital RAW files with this theoretical problem of 12bit and current DSLR sensors. This essentialy boils down to this; when I have to open up the shadows in an image from a DSLR shot in RAW( in a 16bit workflow obviously) why is the acceptable limit to how far I can do this governed by how bad the noise gets before ( way before) I start to see any posterisation.

[Ben_Rubinstein]

I've just been experiencing stairstepping myself, more pronounced in ACR than DPP but definatly there. I can post examples...

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