Digital BW, The Print

Jeff 

Thanks for this.  I am interested in hearing a more direct retort to the
material on Normen's site (I have previously posted the link).  As I read
it, he is quite clear that a greater bit depth provides greater dynamic
range (measured in f-stops or whatever).  He notes, as you do, that noise
affects the "bottom end" of the range and that noise is highly correlated to
pixel size.  (It goes without saying that we would all prefer more larger
pixels, appropriately cooled of course, but digital camera/digital back
design is constrained by both economics and the need to fit current lens
format dynamics.)  But I hear you saying that for a given noise level and
given pixel size, more bit depth makes no difference.  Can you expand on
this a little and if possible correct the information, or my understanding
of the information, on Normen's site.  I suspect the "line of difference"
(so-to-speak) lies somewhere in his assumption "that the darkest useable
zone has 8 levels" and that the number of useable levels is affected by bit
depth, ie he is talking about the limitations imposed by engineering
decisions which affect things post photon capture: that once one have
determined the "size of the well" the ultimately delivered dynamic range is
still affected by bit depth whereas you would say this is not true and that
increasing bit depth is of no value.

Thanks

Steve


> From: Jeff Medkeff <medkeff@...>

> <snip>
> A photosite responding to a photon produces an electron through the
> photoelectric effect. This electron typically gets captured in a
> potential well. (NB: When I say "potential," I mean voltage; when Normal
> Koren says it on that page that was quoted, he refers to that which is
> mathematically possible - this isn't apples and oranges, it is more like
> apples and duckbill platypuses.) The electrons get stored in what
> amounts to a little capacitor right on the sensor. Electrons sitting
> around in this potential well, which do not belong there because they
> came from somewhere other than photons, constitutes noise.
> 
> So the dynamic range of the sensor is defined at the low end by noise -
> spurious electrons hanging out in the potential well (plus amplifier
> static contributed during readout). You have to pick a statistical
> significance that constitutes non-black - "real" black might be defined
> as (say) anything below 25 sigma in the image that gets read out.
> 
> At the high end, the dynamic range of the sensor is defined by clipping.
> A certain number of electrons can be stored in the potential well. If
> you go above that number, the potential (=voltage) gets high enough that
> some electrons find other ways off the sensor than through the readout
> circuits. A capacitor can only hold so much juice.
> 
> If the maximum number of electrons the well can hold is 1,000 (we'll use
> a conveniently small, entirely made-up number), then if you put 1,000
> electrons in the well you read out white. If you try to put 1,001
> electrons in it, you read out white. If you try to fill it with 2,000
> electrons you still get white. The useful dynamic range is between the
> noise floor and the potential well's clipping point.
> 
> Big photosites have big wells. They can store more electrons than a
> small photosite with a small well. But they don't generate any
> additional noise. So the white point gets brighter; the black point
> stays the same. If you are saying to yourself that it sounds like this
> is one way to increase dynamic range, you are right.
> <snip>
>
> <snip>
> 
> Once the exposure is over, the signal is read out. The potential
> (=voltage) in the well is amplified; the amplifier output is sent to an
> analog-to-digital converter. Most ADCs on digital cameras output 12
> bits. So the ADC outputs 4096 analog-to-digital units (ADUs). This gets
> recorded in your raw file.
> 
> The big myth in dynamic range discussions is that somehow, if only you
> could change those ADUs to give you more bits, you'd have more dynamic
> range. Unfortunately the ADUs operate after image capture - they can
> have no effect on the white clipping point of the photosite well.
> 
> Now to be fair, there is a reason for the irrational conclusion that
> bits equals dynamic range: A camera with a larger dynamic range
> *requires* more ADUs to properly sample the signal. Therefore cameras
> with higher dynamic range tend to, on average, output more bits.
> 
> Think of it this way. If you are sampling a brightness ratio of ten, and
> you have 4096 ADUs to do it with, the difference between two adjacent
> ADUs is a ratio of 1.0024. In other words, a part of the scene producing
> an ADU of 100 is 1.002 times brighter than a part of the scene producing
> an ADU of 101. Let's consider this a "small" difference. You can take a
> picture full of subtle tonal differences, and really define a texture
> (like an egg, say) with such a camera.
> 
> If, however, you are sampling a brightness ratio of 100 with 4096 ADUs,
> then the step ratio is (predictably) 1.024. In this new situation there
> is a big real-world brightness difference between a pixel value of 100
> and a pixel value of 101. This leads to posterization; you aren't
> recording enough brightness differences to define a surface. Therefore,
> (most) makers of sensors that have a big dynamic range (usually) provide
> more ADUs in output. Photographers tend to reduce this to the formula
> that the more bits a camera outputs, the more dynamic range it has.
> Unfortunately, using that formula to choose a camera can burn you badly,
> because there are a number of exceptions to the rule. Some cameras
> merely sample a poor dynamic range with excessive precision.
> 
> Now, is all this merely theoretical? No. Much of it can lead to better
> decisions at exposure time and camera-purchase time, just as knowledge
> of tone curves and spectral response of film helps the analog
> photographer at exposure time and film-purchase time. Tone curves and
> spectral response are pretty arcane topics in themselves - many
> beginners succumb to mistaken thinking on these topics. The digital
> medium is different, but no more arcane. Perhaps it is true that
> awareness of its technical underpinnings has not penetrated the
> conventional photographic world very effectively as yet.
> 
> --
> Jeff Medkeff
> Eagle River, Alaska