Vuescan channel multiplier when saving raw files.

[gazza95]

Hi

There have been a couple of previous threads indicating that Vuescan
8.2.17 does not store a true raw file. I am scanning negatives with a
canon fs2710 and saving a 48bit raw file when scanning. By doing this
and having builtin ICC scanner profile set I was expecting the raw file
to be a true raw file. However when I open the tiff file in Photoshop
it is clear that some processing has taken place as the image has a
green tint compared to the orange of a negative. Is this simply the
channel multiplier that Vuescan is applying? W

The main reason for asking this is that the green channel is the first
to get clipped when setting the exposure. I am checking for clipping by
viewing the raw file.

gary

 

[Bart van der Wolf]

SNIP

However when I open the tiff file in Photoshop it is clear that
some processing has taken place as the image has a green
tint compared to the orange of a negative. Is this simply the
channel multiplier that Vuescan is applying? W

VueScan does no postprocessing on the scanner's output data if you
look at the Raw data. If the resulting (Gamma 1 if scanned as
16-bit/channel) image is greenish, it is because the Green channel
received more exposure than the others (assuming slides). It is of
course also possible that e.g. your Colorspace settings are
responsible for the look.

The main reason for asking this is that the green channel is
the first to get clipped when setting the exposure. I am
checking for clipping by viewing the raw file.

Depending on your setup, you might need to either reduce the Green
exposure or total exposure.

Bart

 

[gazza95]

thanks bart.
I was also expecting Vuescan not to be post processing the raw data.
If I scan the negative using a slide film type setting then the raw
data shows the green channel curve having around half the width of the
red channel. While if I use the negative film type setting green curve
is some 20% wider. So this is a significant change. This occurs for
both sRGB and adobe colour spaces.

The Canon fs2710 does not have the ability to adjust the exposure of
individual channels. So either there is a comand telling the scanner it
is scanning a negative and it is automatically changing the gains of
the channels or Vuescan is doing some form of post processing.


gary

 

[Bart van der Wolf]

thanks bart.
I was also expecting Vuescan not to be post processing the
raw data. If I scan the negative using a slide film type setting
then the raw data shows the green channel curve having
around half the width of the red channel. While if I use the
negative film type setting green curve is some 20% wider.
So this is a significant change. This occurs for both sRGB
and adobe colour spaces.

Switching to negative film type will not only invert the image from
negative to positive, but if will also change the exposure ratio
between the RGB channels. This is done to compensate for the
yellow/orange mask of color negative film.

So it is not post-processing but exposure compensation that you are
witnessing. The better Green/Blue exposure will produce higher quality
(lower noise/posterization) scans.

Bart