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zhangyue wrote:
Up to now, I never know what is the definition of Tonal range. All of above as I said is from my understanding. I was trying to learn it as planed but never be able to have time to do so. I must admit I am not knowledgeable enough about PS, print, Gamma, display, etc...
If I know the definition, there may not be any thinking process here. I am open to any correction as I said. but I would not limit myself to 'definition' itself.
Being able to divide signal into finer steps give freedom to post process digitally that we do all the time. We are trying to map 13 bit DR image to 8bit,(though I wonder if this/8bit gonna be the standard forever, 14bits ADC might be enough or having only small benefit, but is it future proof? ) doing that, remap tonal step by 'fill light', pull back 'black', increase 'exposure', 'brightness' etc.. we do treat each digital image by applying non linear gain in digital domain. Having finer step giving us freedom to do so and potentially can do better job with more advanced RAW converter or display which can handle finer tonal step than 256.
Why we have to stick with linear(the same) tonal step? can't we have finer step at higher grey zone? anybody can answer this?
Quantization noise obvious have significant impact on Low DR image than high DR image. Consider we do have option to keep finer step in ADC now, I see no reason we want limit ourselves by truncating the signal.
It is not about closer or further related to anything but do we really gain anything here by having more bit ADC in the loop. I was in the other camp before, but go through this thinking make me want to change and offer the topic to discuss.
The key to me is not do we violate the definition or not, but do we really omit things by default thinking minimal tonal step is the noise level, and discard useful information now just because our display, printer, or even raw converter can't maximize this benefit now. (though the benefit is already here as discussed.)
I want to use again the same example above, 4 bit DR with 2^8 resolution/step. I can potentially apply tone curve to those 2^8 step individually (if I have a advanced Raw converter with Super CPU) to any point I like to create tonal range curve I want. 2^4 step won't give me same performance or freedom.
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The definition actually also looks to the A/D quantization, and it is a very precise description of what you want.
The step size at any point is defined as "The noise OR the quantization step, whichever is larger".
This is the definition used by DXOmark.com, and it says exactly the same as the words Joakim uses here.
http://www.dxomark.com/itext/tech_define_measure/image010.gif
The tonal range can never be greater than the number of quantization steps, so if you use a shallow bit depth you do get a small Tonal Range, also according to the definition.
Now I am not sure if you are talking about the very coarse quantization in a gamma corrected 8 bit image file, or in a raw file with 12-14 bits, or a theorethical case. If you are working in a very small bit space, you of course get problems with posterization, as when you try to edit 8 bit images, but nobody who is discussing Tonal Range here would even think about doing that?
It is also easy to think, since the SNR get better, that noise gets lower as you move up in luminosity. The opposite is the case. At sensor saturation, the noise is enormous compared to what it is in the deep shadows, because the photon shot noise is the square root of the number of photons. Normalized to the same scale: the more light, the more noise, and the larger the TR steps get.
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