Re: NOT scanning negatives as positives


<ljlbox@xxxxxxxxxxxxx> wrote in message news:1126183782.872488.206280@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Bart van der Wolf ha scritto:
SNIP 
Sorry, but I didn't understand a word you said :=)
Let me see how much I've grasped...

With fixed exposure, Photon shot noise has a Poisson
distribution and thus varies as a function of the level of
exposure. The average noise equals the square root of
exposure level.


Here "level of exposure" refers to the *film's* level of exposure.
Correct?

No, film exposure is a given for the scanner.
The scanner's sensors can have a variable integration time, to compensate for film density.

And this noises increases when the picuture's "brightness"
(exposure) increases -- but not as much (just the square
root), so that effectively, if you vary the exposure *of the
CCD*, the signal-to-noise ratio improves. Correct?

To put it in my own words, the darker parts of the film will cause lower CCD exposure levels, but those levels cannot be increased indefinitely by longer integration/exposure times because that would cause clipping of the most transparent film areas. So some CCD sensors will receive a high level of exposure, and some a very low level of exposure.

The lower levels will be relatively more noisy due to the Poisson distribution of photons (e.g. Sqrt(100) = 10 which gives a 100:10 or a 10:1 signal to noise, and say Sqrt(10000) = 100 which gives a 10000:100 or a 100:1 signal to noise which is 10 times better). Therefore it will pay-off to increase the exposure level as high as possible without clipping the most transparent film areas.

> If so, with increased exposure, noise should increase
> just as much as image data. What am I forgetting?

See above. With varying exposure, longer exposure will
increase the dark noise level. There will be spontaneous
noise building up over time (as a function of temperature),
and it is totally independent of any signal reaching the
CCD (hence dark noise).


Now you're talking about a different kind of noise, which
increases linearly with CCD exposure time.

Correct, it is one of the contributing noise sources, one which benefits from the shortest possible integration/exposure time.

This means that, given a certain CCD exposure, every pixel
will be "off" by a random amount between zero and n, where n
is a linear function of exposure time. Correct?


Yep.

There are also other sources of noise in the signal as finally
quantized, but dark noise *and* Photon shot noise both deteriorate
image quality of relatively dense/non-transparent film areas.

However, photon shot noise increases less than dark noise with longer
CCD exposures; the net result is that, by exposing the CCD longer, you
don't gain anything with respect to dark noise, but you get less photon
shot noise (actually, you get *more* photon shot noise, but you also
get *much* more signal - precisely, with exposure time t, you get
k*Sqr(t) noise, but k*t signal).
Correct?

You've got it! The net S/N gain from optimizing (maximizing without clipping) the signal level is larger than the loss from higher dark noise.

In addition, when multi-sampling the film, the random noise decreases as the square root of the number of samples because noise cancels out, while the signal is just the average so S/N improves again. This does require perfect alignment of the samples (e.g. with a single-pass multi-scanning enabled scanner or, much more error prone, by averaging multiple scan-passes).

Bart 

.

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