Ralf Loi wrote:
> , summing 2 or more different scans into one single image in order to
> minimize scan noise.
>
Fernando Gonzalez Gentile wrote:
> Could someone more skilled than myself, explain theory and practice of this
> technique?
>
Fairly simple. You are an audiophile. Imagine a single groove on a
record that doesn't spiral in and with nothing recorded on it at all.
Put the needle on it and turn up the volume. Any hiss or other noise you
hear is system noise. If that noise is truly random and you capture each
rotation, you can add them together. As the number of samples increases,
the apparent noise decreases, until it becomes a tiny DC voltage. It's
still not zero, but is unvarying and thus not heard.
That's system noise, and may indeed be eliminated by summing multiple
samples.
Now imagine that the record has been sitting around and played often. So
there is another kind of noise, source noise. As the location of each
bit of dust, pit, etc. is mostly fixed, summing samples will actually
appear to enhance this noise, by lowering random noise so that the
non-random source noise stands out.
Now imagine sound recorded on such a groove at a very low level. Factors
such as latency in the cutter head and the resilience of vinyl mean that
most of the signal doesn't actually result in a wave in the cut.
However, the loudest peaks do manage to get recorded, so there are
scattered little bits of sound. They aren't random, as they are
amplitude related. However, like the non-random source noise, they rise
out of other, random noise when multiple samples are summed. Without the
missing parts, they appear to be random noise - and - may appear to be
made worse by summing.
That's dynamic range clipping noise.
The situation with film is much the same. Random noise from the scanner
electronics will be reduced by multiple passes. Tiny dust, scratches,
dried processing chemicals, etc. may actually become more prominent with
multiple passes. Fortunately, you have ICE for that.
Perhaps the trickiest situation, however, may the one where
underexposure is sufficient that only relatively scattered bits of the
light reflected from the subject were bright enough to activate the
silver halide in the film grains. As most of the area is not light
activated, development results in an area of clear backing with a
scattering of dark dots. Although theoretically containing information,
like the tops of mountains poking through clouds seen from above, they
don't tell us anything about the obscured part of the landscape.
Again, multiple passes will do nothing whatsoever to recover the missing
detail, only separate the bits from any random noise. With slightly
greater initial exposure, the general nature of the subject may become
discernible. However, the parts clipped in the photographic process are
still not recoverable through any scanning manipulation.
This results in what now appears to be noisy image. And it is. However,
as the noise is dynamic clipping in the source, summing multiple scan
passes will not improve it. It's even possible, depending on scanner
characteristics, for this kind of noise to appear to get worse with
multiple passes. I don't know how common that may be.
It IS possible for software to interpolate missing data based on
surrounding pixels. Various RAW developers for digital camera images
this to different extents. some of it is remarkably effective. Adobe
Camera Raw, for example recovers considerably more natural appearing
highlight detail from a clipped image that Canon's SPP processor.
Whether there is such software for film scans, I don't know.
I have written all of the above assuming we are talking about negative
film. Just to be clear, it also applies to reversal (slide) film. In
reversal film processing, the negative image is first developed, at
which any noise as a result of clipping at either highlight or shadow
level is established. Later processing steps chemically reverse the
luminosity of the image, creating a positive image. However, that
chemical process increases contrast, actually decreasing the dynamic
range of the original negative stage, and clipping more luminance
extremes than the first development step. That's why slides have less
dynamic range than negative films.
> My scanner is a Nikon 4000ED. I used to scan each slide at the maximum number
> of passes the machine is capable of
In light of the above, I can't see any benefit to multiple passes beyond
what may be necessary to reduce system noise. It should be easy enough
to find that by scanning a completely dark piece of film and observing
the pixel level with various numbers of passes.
As the 4000ED is a very good scanner, with large dynamic range, I
wouldn't be surprised to find that the number of passes needed for
maximum system noise reduction is small.
> , until I read Ken's post on June 2 when he says five passes are more than
> enough.
Ken says nothing whatsoever in that post about noise reduction. In fact,
he says:
"BTW, why five-pass? Because the extra passes help define the shape of the
grain."
> I don't think I will change my habits, though :-)
>
A compulsive behavior disorder? Ken says"
"I'm doing up to FIVE passes. Is that necessary for all images? Hardly. Get
real. .... I'd slice my wrists if I had to do five pass scanning on every
image."
> But there are scans which are problematic, those slides which have details in
> the shadows and careful adjustment of curves and luminosity and USM don't
> seem to resolve accurately.
Whether the problem is simply missing data or less than perfect PS
technique, I can't tell from here.
> I was thinking about doing something like HDR scanning, merging the resulting
> files using CS3 - but I don't know if this makes any sense.
>
It will make no difference if the detail is indeed clipped. Using HDR
software to create extreme tonal shifts and LCE, as so many people do
will, however result in a very different looking image. Combining
multiple exposures taken when the subject exceeds the dynamic range of
the film would certainly help create a natural image with greater
dynamic range.
Moose
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