Digital BW, The Print

Jeff

I understand, at least in layman's terms, this.  Let me then rephrase my
initial statement into a question:  do you feel that for a given Canon
sensor (say the 1Ds MK II sensor) there is anything to be gained from
increasing the bit depth of the ADC?  I, like Paul, am not going to wait for
a fundamental transform to occur in pixel design and understand that for a
given lens format there is a limit to effective sensor size and that within
that constraint we play off resolution against pixel size.  (I took the jump
to the 1Ds MK II because, rightly or wrongly, I thought that we had reached
close to the end of the pixel race in 35mm format - many more pixels within
the 35mm format constraint would lead to too small pixel wells and lower
dynamic range and likely increased noise.)  I was always puzzled, though,
particularly having read the digital tonality material on Normen's site, why
a camera manufacturer would not deliver 16 bits to the user and hence, as I
understand it, improve the usefulness (read "editability") of those dark
areas just above the noise threshold.  I understand that if you have a
bigger dynamic range at the sensor, say from a MF digital back with larger
pixel wells,  you are forced into a higher bit depth in the ADC because else
that range, particularly the low end, would more readily get torn apart by
the RAW converter and heavy editing.  But the question remains as to whether
12 is enough/optimal for current 35mm sensors and whether a significant
improvement in usefulness can be achieved by increasing bit depth to, say,
16.



Steve


(BTW an extension of your discussion is that as 645 sensors increase their
pixel count and reduce their pixel well size then they can reduce the bit
depth....I would be very surprised to see this happen.)


> From: Jeff Medkeff <medkeff@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Sun, 03 Jul 2005 22:18:18 -0800
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Subject: Re: [Digital BW] Artifacts with Digital images
> 
> Roy Harrington wrote:
> 
> 
> 
>> Basically you increase the sensor dynamic range by either: reducing the noise
>> at the low end and/or increasing the clip point at the high end.  The bits of
>> the A/D have to fine enough to take advantage of the reduced noise at the
>> low end and coarse enough to not get clipped at the high end.
> 
> It sounds to me as though we are approaching this from two different
> perspectives. When you say the bits have to be coarse enough to take
> advantage of reduced noise, and fine enough not to clip, it sounds to me
> as though you are saying something about the digital sampling of the
> signal being read off the sensor. And if I understand correctly you are
> dead right - there is no point to making the luminous flux density delta
> per ADU so coarse that the 100 (or whatever) highest ADU values all clip
> to white. Not only do you throw away values, but you also increase the
> chances of posterization in the resulting digital image.
> 
> You state correctly, above, that to increase sensor dynamic range, you
> can reduce noise. You also say you can increase the clip point at the
> high end. I'm familiar with using the term "clip" in reference to
> digital sampling, so it appears as though what you are saying is that to
> increase dynamic range you should take your highest ADU - 4096, say -
> and calibrate that to what is detected from a higher luminous intensity
> (luminous flux per unit solid angle) from a scene at a given EV. Maybe
> that is what you are suggesting, or maybe not - that's what I'm hearing.
> 
> Either way: Doing that won't work.
> 
> Well, I admit it will work in one special case: It will work when your
> sensor's highest ADU value represents a number of electrons that is less
> than the full well potential of the sensor. In your terms above, if the
> "bits of the A/D" are a bit beyond "fine enough," and are in fact too
> fine, then your digital image won't have the dynamic range it could have
> had if there were more bits available at the same sampling rate. By
> sampling rate, I mean electrons per ADU.
> 
> But if your highest ADU value already represents a number of electrons
> that is equal to the full well potential of the sensor, then adding bits
> at the *same* sampling rate buys you nothing. This would be too "coarse"
> sampling, as you put it, and all those added ADU values would be clipped.
> 
> Increasing the sampling rate (reducing electrons per ADU) while
> increasing bits *can* solve this clipping problem - and I'd like to have
> this in all my cameras. Unfortunately it will not result in greater
> dynamic range. The reason is that the maximum luminous intensity and the
> minimum noise recorded by the sensor are both limited at the analog stage.
> 
> The analog stage is mostly what I've been talking about. We are always,
> always, always limited by the photosite well. A well can hold between
> zero and n electrons. If you exceed n, and try to stuff n+x electrons
> into the well, an average of x electrons will be lost to other parts of
> the sensor. This is because the well achieves enough voltage to jump the
> resistant gap between it and another capacitor or some convenient path
> to ground. In either case, these electrons have overflowed their bucket,
> and are no longer present in the well when the sensor is read out.
> 
> In this situation the well is said to be "saturated." This sometimes
> results in blooming, but it does not result in clipping as I understand
> the term, because this has happened before any sampling of the well is
> made; there is no chance for the amplifier output to distort or the ADC
> to clip as these electrons never get there.
> 
> So a well can hold between 0 and n electrons. For a big well, n is a
> larger number; for a small well, n is a smaller number. Quantum
> efficiency determines how many electrons end up in the well, as a
> percentage of quanta of photons that are incident upon the photoelectric
> surface.
> 
> If the sensor QE is 50%, and 1k photons strike the sensor, then 500
> electrons (on average) are dumped into the photosite well. The number of
> photons striking the sensor is determined by the luminous intensity of
> the scene. So if we double the EV, we will have 2k photons striking the
> sensor and 1k electrons in the well. If we double it again, we'll have
> 4k photons and 2k electrons.
> 
> Suppose the well capacity is 1k electrons. We've tried to stuff twice
> that many into the well. Half those electrons will find some other way
> out of the well, than through the readout amplifiers. Does adding bits
> to the sampling do anything to recover the lost 1k electrons?
> 
> The correct answer to this question is "no." The analog limitation of
> well depth is fundamental and exists because those lost electrons are
> not amplified and never affect the signal that the ADCs see.
> 
> And zero is a possible number of electrons in the well. This does not
> lend itself to useful representation with a proportion, which is
> probably one reason why dynamic range is specified on sensor spec sheets
> with luminous flux density and electrons of noise, rather than a ratio.
> 
> To increase the dynamic range of the sensor, we have to either reduce
> noise, or increase the saturation point of the well. The latter means
> increasing the number of electrons it can hold, because that correlates
> linearly with the maximum brightness in the scene that the sensor can
> usefully record.
> 
> What I originally started this discussion with, was a statement that
> photographers should beware the abuse of bit depth specifications. It
> does not always tell you anything about the ability to record the
> dynamic range of a scene, because no matter how many bits you have, the
> highest n ADUs could all be representing saturated photosite wells that
> contain no useful information about the scene you've just photographed.
> I think bit rate is going to be the next great digital camera scam, once
> everybody gets over the megapixel fetish - it already appears to be
> happening in the digital back realm.