Why the difficulty of producing an acceptable positive from C41 film in the digital conversion?

Steve@f8 said:

it would seem (before actually trying to do it yourself) to be a ‘simple’ task of inversion and adjusting the sliders to taste, colours, contrast, brightness

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koraks said:

As far as I'm concerned, that's pretty much what it boils down to, yes. I generally don't touch contrast or brightness sliders and only do curve adjustments.

Flipped – doing color negative inversions manually – Koraks tinkers

tinker.koraks.nl

I find it easier if I scan several frames in one go (flatbed) and do an inversion + adjustment curve on all of them at once. Then scan all frames of the same film with the same settings (without any autocorrection enabled in the scanning software) and apply the same curve adjustment to the whole batch. For me, this is the quickest and most consistent way of doing it. YMMV and all that.

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loccdor said:

You didn't mention if you're doing this digitization with a camera or a scanner.

I do it with a camera mainly, and one of the things I've noticed that orange-masked film is particularly affected by is stray light from not masking your backlight source perfectly. With slide film and a white light source, stray light is going to have the same color temperature as the light going through your film, and the more contrasty brightness range will mitigate it further. But with orange mask C-41, any stray light which is not being filtered through the orange mask is going to result in a different color temperature flare which may be near-impossible to mitigate in post processing. C-41 also has less contrast within each channel which means the effect of a flare will be greater since you are magnifying the contrast.

Assuming stray light is not the issue (you've masked and hooded things well, or are using a scanner), then if you are digitizing C-41 in 16-bits-per-channel it's important to look at the individual R-G-B histogram and make sure none of the channels are getting truncated. That will give you a RAW or TIF that you can work well with.

Having surmounted those two C-41 obstacles, you can invert, and then set your black point to the minimum of each of the R-G-B channels individually, and your white point to the maximum of the channels, in order to normalize the image and stretch the values over the full range available.

Then, you can pull the R, G, B curves up or down individually until you stop seeing unnatural color casts (unless desired for the effect). This is the part that takes a while if done manually, as I do in Gimp. Sometimes 5-10 minutes to get it right in an image I really care about. It does take practice to get the feel for what colors look natural as the eye adjusts. Usually pulling from the middle of the curves with a single point is sufficient, except in complicated cases. When you work on a whole roll of a particular type of film, you tend to see that you're making similar color adjustments on every image, unless some are badly under or over exposed. So it gets faster as you practice.

Of course, you can get more advanced than me and use software specifically designed to handle color negative, which may streamline some of these steps in some cases, at the loss of some manual control.

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Osmdesat said:

While orange mask can be always compensated in post-process, I think much better is to pre-balance the negative image already using lightsource of appropriate colour or to use appropriate filter (I mean optical filter you put on the lens, not "filter" in Instagram app or whatever).
This way all the color channels will be exposed equally. If you use normal white light as backlight, the green and blue channels will always come out underexposed and you will have to amplify them in picture editor to balance the colors. So for me the starting point would be to have negative pre-balanced optically. Further digital balancing leads to loss of quality.
You can experiment with blue filter on the lens or very cold white LEDs like 9000K or so, however this will not be ideal.
Better to have gelatine CMY filters for color enlarging. you can pre-filter the backlight with suitable combination of theirs. If you have color enlarger with dichroic filter head, you can use this as a light source. Depends on your possibilities.
The desired compensating light will gonna have blueish-greenish tint.
I personally use diy RGB variable backlight for these purposes, but given that color LEDs produce narrow-band light, the colors in resulting scan will be somewhat off and they will have to be pre-processed for correct colorspace.

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xkaes said:

Steve, we all feel your pain. One would think that a simple solution would have been created a long time ago. Film scanners do it instantaneously, so why isn't there software available to do the same damn thing?

There is, and it's been discussed a lot on this FORUM -- do a SEARCH for previous threads (upper right hand corner of this screen).

That being said, the software available has its PROS & CONS, from cost to complexity (see Post #2) to accuracy.

Perhaps your resurrecting this discussion will uncover some recent advances -- if not perfect solutions.

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Steve@f8 said:

I only have a bog standard light panel and I guess it’s somewhere in the region 5000 to 5500K. No filters at my disposal either, but your observations and tips are really helpful. Thank you.

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Steve@f8 said:

One thing in particular I picked up on is the way you deal with the dreaded orange mask. It doesn’t appear that you’re adjusting the white balance by using the dropper sampler on the film base, or on a neutral tone (if there is one, and if so can be recognised under the orange mask). instead you’re looking for a secondary peak in the RGB channels, predominantly red. Is this because of GIMP or for preference or better outcome?

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