As it appears, I forgot to mention many important things. LCE will indeed not work well with unbalanced negatives, but there is an objective explanation, and a fix.
By unbalanced negatives we should understand those exposed under a light that differs significantly from the one the film is rated for. For example, consider we shoot a winter scene in the Northern direction on Ektar 100, which is rated for “daylight”. The real temperature for such scene would be way higher. Not unexpectedly, what we get after the LCE conversion is a much stronger cool cast. This is similar to what we’d get with a digital camera set to e.g. 5500K, meaning, the behaviour is predictable, and maybe even useful.
Here is such an example. The correction is exactly the same, except for the dynamic range. Same exact Hue/Saturation adjustment applied here, yet it is obvious it is not enough.
This was shot in February around 13:40, the very end of the afternoon, beginning of the sunset. The sunlight color was probably still pretty “daylight”. The camera is looking North so all the shadows and everything not in direct sunlight should be quite cool (or hot in Kelvin terms).
The fix is either going further with the Hue/Saturation adjustment, and I did that before, or treating the cyan tint as an uncorrected WB, and correct it as such. Here is a cool thing. See what happens if we just change the WB (well, actually also reduce the exposure a bit as in the Camera Raw changing the WB blows the highlights):
Ain’t it nice? So far the theory holds. The correction is still global so should be automatable.
Now that I think of it, maybe the amount of the “residual” cyan cast can be measured and provide an idea of the real scene white balance, a piece of info not otherwise obtainable from the frame.
As an extra, here is what happens when we replace the initial Hue/Saturation with a stronger WB correction, followed by a Hue/Saturation adjustment to reduce the cyan in the highlights:
Note that this may be less representative as this state hits the color space boundaries and the colors change significantly when converted from ProPhoto RGB to sRGB. Also note but disregard the weird shades of the snow in the foreground. This Ektar is expired and that’s how it manifests, the effect is exaggerated by the conversion to sRGB , meaning it’s a different issue.
Another new observation is the Ektar curves. Note how B is not actually parallel to G and R. Maybe that contributes to the extra cyan in the highlights.
Also note the Status M
densitometry, as opposed to Printed Density
. This does makes sense, as this is what the Status M
is for, measuring the film response on its own. The whole system of film/paper should be measured with Printed Density
, but not the film individually. Nevertheless, this needs more thinking.
The thing is, film curves actually are sigmoids as well, its just their shoulder is not shown on the charts. I remember reading about it in one of the books, but can’t remember where.
On this chart the channels more or less coincide, meaning the image is balanced. Can it mean a finished print measured for the color reproduction? I’m not sure I understand what does it tell us, could you please explain?