Hi Manfred,
Thanks for your replies to my queries.
You're right, I had not thought that far aheadthe scans and restorations are going to potentially happen over a period of years. He is also looking at using two different scanners for the process.
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I have noticed slight colour differences in the output of my two scanners (same brand, different model).
In respect of bit depth for scans, to clarify, if I were doing it, the very first task upon opening the 8 bits/channel original scan would be to convert it to a 16 bit workspace so that I don't lose any data when post processing - and (of course), save it with a different file name - rule #1; NEVER overwrite an original file.I this kind of work, there can be some fairly heavy duty adjustments, to bring back the colours by working on a channel by channel basis. These adjustments are definitely more extreme than what one would encounter in normal day-to-day PP work from a digital camera capture (unless the exposure was totally blown). I found I had to spend a fair bit of time cleaning up artifacts, that I suspect might have been the result of using an 8-bit image. I expect that the issues were likely due to pushing the PP software to extremes and building on additive errors of the various incremental steps I was taking.
I suspect that an 8-bit jpeg will be more than sufficient for 95% of the images I've worked on. The problem is that other 5%, and frankly I can't tell by just looking at the image if problems are going to arise in the restoration work.
AFAIK, almost any manipulation of say, exposure values, is going to result in the resultant math on given pixels exceeding 8 bit depth. If we just do a few operations, it probably won't be noticeable 95% of the time, but as you also say, you cannot visually predict which ones will be 'compromised' by reduced bit depth, so safer to work 16 bit/channel, as indeed was my intention.
True enough, and it may be simpler (and safer if likely to forget to up-convert bit depth), to start as you mean to go on.With storage being relatively cheap, I would tend to scan to TIFF and stick with 16-bit sRGB data.
I just seemed to be in a "why do we do that?" mood last night
The artefacts were probably because of the capture jpg coding process (and compounded by any subsequent jpg processing that might have been done) rather than bit depth, especially if they were 'edge related' - IOW the change between adjacent image content that is dark and light in tone. That said; if the artefacts were posterisation, then yes, it was a bit depth issue. Chances are, both will apply though!I ran into a situation early last year where someone asked me to redo a colour wedding photo that was both faded and stained and all they had was someones jpeg scan for me to work with (no original available). If I recall correctly, the scan was using sRGB.
I this kind of work, there can be some fairly heavy duty adjustments, to bring back the colours by working on a channel by channel basis. These adjustments are definitely more extreme than what one would encounter in normal day-to-day PP work from a digital camera capture (unless the exposure was totally blown). I found I had to spend a fair bit of time cleaning up artifacts, that I suspect might have been the result of using an 8-bit image. I expect that the issues were likely due to pushing the PP software to extremes and building on additive errors of the various incremental steps I was taking.
As long as the image is being edited in 16 bit depth and remains open in the image editor, errors of bit depth and jpg artefacts should not accumulate, even if we repeatedly Save (as jpg). The artefacts should only become additive if the image is repeatedly cycled through "saved > closed > opened > edited > saved" operations. The most damage is caused by the conversion to 8 bit and the jpg coding process, but this only occurs (with best practice) to the exported file, not what is in the image editor's memory. I made this point recently in another thread here (by a different member) on photographing their family archive of documents and photos.
A related analogy:
I guess many of us have photocopied documents in our lifetime - if we need 10 copies, we put the original on the glass and photocopy it 10 times, we don't do one, use that as the original for the next copy, and so on, because we know that the tenth copy will be 'dire' - but if we repeatedly cycled through "jpg saved > file closed > jpg opened > edited > jpg saved" operations, that's what we're doing.
Thanks, Dave