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[OM] Re: minimizing scan noise - was: How I did this?

Subject: [OM] Re: minimizing scan noise - was: How I did this?
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Sat, 07 Jun 2008 08:11:51 -0400
Thanks for the noise treatise.  Well written and easy to understand.

Chuck Norcutt

Moose wrote:
> 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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