Hello Kennedy,
first I´d like to say, that I´ve not studied physics or something
similar, so my untechnical (wrong use of terms?) language might led to
misunderstanding. A problem often found between profesionals and
laymans. I think, that there are some popular myth about the
significance of "bits". I always get this impression, if there is a
discusion about scaner, like recently in our list. For this reason, I
think it´s worth to discus this publicly in the list, if you don´t mind
please forward our discusion to the list.
Kennedy wrote:
> I think you are using a different definition of dynamic range from me.
> Normally it is defined in dB, as 20log(saturation limit/noise level)
> with both levels expressed in volts, or 10log(.....) if levels are in
> power or intensity. That definition certainly gives far higher figures
> than the 1.8 - 3.8 you are quoting, which sounds more like the gamma
> response than anything else I can think of. What are you defining as
> dynamic range?
I´m relating to the log (dec) relative optical density or brightness,
used in print (photographic and paperbased), based on the brightest
(paper, lightsource) and darkest spot (dye) of an picture.
> Whilst the number of bits used in the ADC is unrelated to the dynamic
> range of a CCD, using less bits than the CCD is capable of simply limits
> the dynamic range further, whilst increasing the bits simply quantises
> noise - increasing cost without any increase in performance.
Logic, a 1bit device would produce either black or white with nothing in
between, but a high bit device (f.ex. 12bit) would not be of much use if
the combination of CCD, AD converter and optics did not allow to produce
more than 1024 shades (8bit).
> Also recall that the figures I gave were for two-dimensional CCD's,
> where the storage capacity is limited by the space available under each
> pixel. In the examples that you have quoted, a linear CCD is used where
> the storage capacity can be much larger since it is ultimately limited
> by the mean free path in the substrate if sufficiently large gates are
> fabricated. Linear CCD's with storage capacities of 50-100 million
> carriers are not unknown, giving a dynamic range up to 10,000 and
> requiring at least 14 bits to fully quantise into the noise floor.
>
> Of course, the time required to scan a frame with such devices makes
> them totally impractical for use in a digital camera back - unless you
> only want to make pictures with exposure times of the order of several
> tens of seconds.
You wrote earlier:
> Astronomical units - as Lee will doubless confirm - achieve higher
> dynamica ranges by reading the CCD out numerous times, converting the
> signal to digital form and accumulating the result. This increases the
> effective photon noise limit by a factor of the square root of the
> number of integrations, but pretty soon this becomes dominated by other
> effects - particularly dark current noise etc. Whilst such multiple
> integrations are fine in the relatively stable astronomic field, they
> would not be suitable for normal photography since the subject movement
> would corrupt the image.
Didn´t Nikon use(d) this technique with there Coolscans? I´d remember to
have read, that the scaning time is relative to the slides density.
Greetings
Richard
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers
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