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Re: scanner quality when profiling?



At 11:27 PM 2/18/2000 -0500, Bernard Epstein wrote:
>  Steve:   You make an interesting point: the scanner's tube deteriorates 
>with age, like a room fluorescent tube? Would light output decrease 
>precipitously rather than gradually? .......I scanned a series of about 40 Fuji 
>slides on my 2-year-old Nikon LS-20 via Vuescan. All the slides (Velvia or 
>Sensia) were of comparable density, although some were taken at dusk 
>or night. At the beginning of the sessions, I was getting very good results with 
>a gamma of about 2.2 to 2.4 and occasionally a bit less; these images required 
>very little tweaking in PS to get them as close to the original as I could hope for 
>(at least as they were displayed on my monitor). Then I noticed I was having to 
>increase gamma to the 3.6-3.8 range and even 4.0 to avoid very dark raw scans. 
> <SNIP> .....(the 50+ MB raw files would start eating up too much
>storage space anyway, so I throw away the raw scans after converting to
>24-bit). 

I usually throw away my raw high bit scan files afterward also, but in this
case the easy way to test this idea would have been to re-scan one of the
earlier images and compare the two files.  An even better possibility (not
much use to you now, admittedly) would be to have a reference scan from
some type of test target that you could re-scan periodically and check.
(I find it much easier to compare the values in a well-defined block in a 
color chart than to find the exact same spot in most normal images.)

Your suspicions are probably correct that the "gamma" settings in the
two scanner programs are unlikely to be directly comparable.

Gary Hunt <glh@srv.net>

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