nothing is broken. your scans are picking up more noise in the extra bits. there is much less run length correlation in the scan lines in a 16-bit image compared to an 8 bit image. this tells you how much random noise is present because of film grain and noise in the scanner. you should try the same scan in multipass mode (assuming your scanner supports it) to see if the files compress better or worse. try at least 8 passes. if your files then compress better, you have shown that your scanner has mostly noise in the least significant bits of the scanner A/D converter in 16-bit mode.
That's interesting - and I ran some tests that confirmed it. I scanned the same slide in Vuescan, once with one pass, once with 16 passes. Both uncompressed scans were 138,016 KB. But, after saving them as 'compressed' tiffs, the single pass scan ballooned to 149,736 kb, while the multi pass scan only increased to 139,116 kb. The same scan, scanned on Canon's FilmGet software, went from 138,001 kb uncompressed, to 153,917 kb compressed...
I guess this raises the question - should I even bother with 16 bit scans? Is there any way to really test the difference in quality? This makes me wonder if 16 bit files are a waste of time and disk space...
- MCC
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Mark Cassino Photography
Kalamazoo, MI
http://www.markcassino.com
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