Clipping in Vuescan different than in Photoshop?

In Vuescan, I'll start by setting color balance to manual and set wp
and bp both to .01% and output a 16 bit greyscale tiff file. Then,
I'll set wp and bp back to 0, set color balance to none, and output a
second, 16 bit greyscale file. I change nothing else.

In Photoshop I open the second file and clip wp and bp .01%. I then
open the first file as well, the one clipped in Vuescan. The Vuescan
file is much flatter, with longer toe. How come?

"Mendel Leisk" <leisk@telus.net> wrote in message
news:e59b7450.0401261907.390d8678@posting.google.com...
> In Vuescan, I'll start by setting color balance to manual and set wp
> and bp both to .01% and output a 16 bit greyscale tiff file. Then,
> I'll set wp and bp back to 0, set color balance to none, and output a
> second, 16 bit greyscale file. I change nothing else.

You are changing 2 parameters here, Black/White point AND color balancing.

> In Photoshop I open the second file and clip wp and bp .01%. I then
> open the first file as well, the one clipped in Vuescan. The Vuescan
> file is much flatter, with longer toe. How come?

So VueScan clipped less. Assuming PS uses 2 decimal accuracy, it may also be
the result of PS actually being a 15-bit/channel application.

Bart

Hi Bart, thanks for the tips. So it appears that switching from "none"
to "manual" does more than just activate the ability to clip, it is
perhaps applying a curve, as defined in the film profile you have
selected (in my case, TMax400 w/ D76ci:.55).

I found (with one particular image) that to closely match a "none"
output which had brightness 1.0 and which I subsequently clipped .01
both ends in photoshop, with "manual" setting I needed to set
brightness to .85~ and clip .02 both ends in Vuescan.

"Mendel Leisk" <leisk@telus.net> wrote in message
news:e59b7450.0401271217.58824bb7@posting.google.com...
> Hi Bart, thanks for the tips. So it appears that switching from "none"
> to "manual" does more than just activate the ability to clip, it is
> perhaps applying a curve, as defined in the film profile you have
> selected (in my case, TMax400 w/ D76ci:.55).

Correct, in the help files it says:
"None
The black and white points aren't used at all, and the image is only
corrected for the CCD's color response (if the Media type option is set to
Image) or by the film's color response. This image is gamma corrected."

Bart

Sort of off-topic, but what the heck, it's my thread:

I just upgraded from 7.6.53 to 7.6.55 and I see the histogram is on
the move again, for black and white negative setting, atleast.

For the first time, in 7.6.53, I was not getting a cronic uptick at
the black end of histogram, often there was even a bit of space. This
was without using advance workflow.

Now with 7.6.53 the uptick is back. BUT, I found if I just tick "lock
film base color" box (without going through advanced workflow), and
then in color tab set each of the added boxes (film base color red,
green and blue), to 1.0, I eliminate the uptick, and seem awful darn
close to the 7.6.53 output.

Comparing histograms of before and after, in photoshop, confirms this.
The histogram shapes appear identical, and the statistics, Mean, Std.
Dev. and Median, are EXTREMELY close.