On 2017-03-28 16:01, Remco Viëtor wrote:
On mardi 28 mars 2017 14:49:09 CEST Per Östlund wrote:
> Hi,
>
> I've digitized my father's collection of old slides with the camera +
> macro lens + light table method. Most of the images look fine, but some
> images with certain blue hues are giving me a headache. I'm guessing the
> issue is that the light table I built used LEDs with a poor color
> spectrum for the task, causing the blue channel to blow out.
>
> As soon as I touch e.g. the levels module I get the usual issue that
> very saturated blues turn black. Using the gamut clipping option in the
> input color profile module fixes that particular issue, but doesn't do
> anything for the colors otherwise.
>
> The best solution I've managed to come up with so far has been to set
> the input color profile to "linear Rec709 RGB" which heavily desaturates
> the image, and then increasing the saturation a lot with the color
> correction module. Hardly an optimal solution, but with some tweaking I
> can usually get the colors to look approximately the same except that
> the blues don't clip anymore.
>
> This method can require quite a lot of tweaking to look good though, so
> my question is if someone knows of a better way to handle this issue
> (besides reshooting with a better light source). Here's an example
> RAW-file (don't mind the blur, that's from the original slide):
> https://www.dropbox.com/s/6mshjycwm4n10xa/img_014.ARW?dl=0
First the good news: as far as I can see you have no blown channels in
the raw file, even the blues stay within range of the sensor. The bad
news is that with the default settings, a large part of the image
gives 'out-of-gamut' warnings. And to be honest, the image looks
over-saturated.
So what I tried was:
- set base curve to 'leica' (less aggressive than the Sony default,
esp. in the highlights).
- set gamut clipping to 'adobeRGB' (or sRGB).
then I used two copies of 'contrast brightness saturation',
- the first for a global saturation correction of -0.2,
- the second was at saturation -0.35, with a parametric mask limiting
the effect to the blue colors: 'b' channel from -128/-128 (full left)
to -29/-3.
Thanks, I tried your settings, but to my eyes it makes the image too
dark without really fixing the issue with the blues.
If all slides are similar, you could set this up for one slide and the
create a style to apply those settings as a base to others.
Unfortunately not, I already have a style which fixes the issue
somewhat, but usually have to spend a lot of time fixing individual images.
But would it be possible that the original slides have faded a bit
over time, and especially the red dye?
Very possible, the example slide is about 40 years old. Unfortunately I
don't have access to the slides at the moment.
Good luck,
Remco
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