16-bit files are much less prone to posterization than 8-bit files if one does a lot of tonal correction on the file in Photoshop. A while back I asked for suggestions on how to deal with some 8-bit drum scans of 12X20" negatives which I did not want to re-scan. I received some good adivice on this forum and have already applied it to several of the scans.
There is also another issue. Suppose you have a lot of existing 8-bit RGB files, or own a drum scanner or high-end flatbed that only scans in 8-bit. I asked this question on the ScanHi-End Group on Yahoo, and received a number of suggestions for converting 8-bit to 16-bit. The system that has worked best for me in scanning LF B&W negatives is this.
1. Scan in 8-bit RGB, doing as many tonal corrections as possible *before the scan." The reason for this is that many drum and high-end flatbed actually scan in 14-bit or 16-bit, and then convert to 8-bit for save. If you do as many corrections as possible before the scan these are done during the scan in either 14-bit or 16-bit.
2. After the scan, open the file in Photoshop and then before doing anything else, convert the file to 16-bit RGB. If you are working with a LF negative this is going to give you a huge file, so be patient.
3. Next, change the file from 16-bit RGB to 16-bit Grayscale, then save. You can do further corrections on this file as if it had been originally scanned in 16-bit.
If you look at the histogram after converting to 16-bit grayscale you will see that it is much smoother than an 8-bit grayscale file, with just a few spikes. Not as smooth as a file scanned originally in 16-bit, but not far off, and so far I have not seen any posterization with this method.
This method could also be extended to any existing 8-bit RGB files.
Sandy King
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