Setting black/white points in Vuescan

[cubilcle281]

Hi,

Is there an easy way to automatically set black/white points in
Vuescan?

Under the color tab (advanced mode) there is an option for 'auto
levels' but the users guide says that this gets set to 0 by default.

Am I missing something obvious? I would prefer just to do a raw scan
and fix in p/shop but I am limited to using an 8bit/channel scanner so
I need to do as much pre-processing as possible. I am using a
Sprintscan 35+ which does 10bits internal/8bits output so I figure the
streching of the contrast would be done internally resulting in a
higher quality final scan. The reason I am stuck with the Polaroid is
that I am scanning an old film that is slightly larger than 35mm, and
almost every modern scanner chops off the top & bottom. The only other
option is a MF scanner which is outside my pricerange.

Thanks,

C

 

[Roger S.]

You can set the white and black point clipping percentage to a
reasonable value (maybe white point to .1 to .3%), however I'm not sure
this is going to help with your internal/output issue.

 

[cubilcle281]

Thanks. For some reason vuescan doesn't appear to calculate the points
automatically though (although, it does for my flatbed scanner).

As it turns out, it does not appear to help the 10/8 bit issue anyway,
so I will probably end up fixing things in photoshop.

C

 

[Bart van der Wolf]

Thanks. For some reason vuescan doesn't appear to calculate the
points automatically though (although, it does for my flatbed
scanner).

Have you tried "Neutral" color balance? Be aware that not all channels
need to fill the full histogram range. If certain highlights lack a
certain color then filling the range (like auto levels) will introduce
a color cast, Neutral will maintain the relative balance.

As it turns out, it does not appear to help the 10/8 bit issue
anyway, so I will probably end up fixing things in photoshop.

Depending on the driver settings, some scanners are fed a lookup table
for gamma, and respond by returning gamma adjusted data. That means
that the scan is indeed done in 10-bit/channel, and gamma adjusted
output is what results, 'nothing' lost. To avoid further chances on
posterization, just change e.g. Photoshop's Image mode to 16-b/ch
before postprocessing, and (depending on the PS version) turn
dithering ON with color management. to save space, you can change the
mode back to 8-b/ch after you've done your retouching, etc..

When you instruct VueScan to scan with an adjusted gamma, that is what
I suppose will be the look-up table fed to the scanners that allow
that. Besides focusing and optimizing exposure time, that basically is
all that's required for an optimal scan quality. Scanning to the
scanner's colorspace (if you have a profile), will then allow to
convert to another (smaller- or output) colorspace, and the dithering
will avoid posterization between similar colors in smooth gradients.

Bart