Re: [APML]: Scanners (using Gamma settings)

[Chuck Vaughn <aa6g@aa6g.org>]

johrt@cableregina.com wrote:
> 
> While not as dramatic, the same principle holds for contrast and
> brightness. Adjusting them digitally is frequently a lossy process.
> They should be adjusted in the scanner as well,  When you do that,
> adjust the brightness until the the the image appears centered and then
> adjust contrast until darkest and brightest areas just register (ie dark
> =0 and bright= amx), then adjust the gamma as you wish.
> 
> This probably is clear as mud!  Also, older and less expensive scanners
> may noy have all or any of the "adjustments".  Still do play with
> adjusting the scanner.  It is the least lossy method of getting a
> quality image.
> 
You pretty much described the procedure I experimentally determined
produced the best looking scans with my slide scanner. I always try 
to perform most of the adjustments on the prescan image. The output
would be miserable if I didn't. I rarely find it necessary to adjust
the gamma. A value of 1.0 is normally fine.

My scanner software also has something called "Exposure" adjustment
which turns out to be very important. When scanning astro negatives it
nearly always sets the exposure too dark. It's easy to see what it does
to the image but I don't really understand what it's doing with the
data.
The range of adjustment is from -2.0 to +2.0. It acts just like over or
under exposing a real photo. Plus makes the image brighter and minus
makes
it darker.

The procedure that seems to work the best is to let it do the prescan
and then reset contrast/brightness/exposure to 0.0 and gamma to 1.0.
Adjust the exposure so that the image looks overexposed but shows all
the faintest (darkest) details (nebulosity in this case). Next adjust
the contrast and brightness so that the image looks right. Do the
real scan and make small adjustments in Photoshop.

Chuck Vaughn
<aa6g@aa6g.org>