The best way to color correct C-41 negative film scans
Reliably color correcting your scanned film negatives is an exercise in frustration. Not anymore! I've created a set of Photoshop actions that completely automates the process and will get your scanned images looking great with minimal effort and while preserving maximum image quality.
Anyone who scans their own C-41 color negatives at home has felt the pain of trying to get the correct color and contrast out of their photos. I too have felt this pain, and I've spent hours reading and researching different approaches to get the most reliable results with the least amount of effort all while preserving maximum image quality. I've figured it out and packaged a set of automated Photoshop actions for you to use that will get you a perfect color corrected image 99-100% of the time.
You can use any scanning software you like, as long as you save your scans as positive images in 48 bit mode (16 bit per channel) with all edits and scanning enhancement options turned off. The only other software you need is Photoshop.
Update April 19 2016: I tweaked the actions a bit because I received complaints that the resulting shadows were too grainy. I also included an action for black and white scans.
Download the Photoshop actions here. If you don't know how to install Photoshop actions, Google how to do it whether you're on OSX or Windows.
With your positive scanned image open in Photoshop, you need to crop out any frame borders before running the action. I also recommend you clone out any dust or scratches first, too, because anything pure white will throw it off. Google how to run Photoshop actions if you aren't familiar with them.
All of the edits done by the actions are non-destructive, so you can tweak or undo changes as you desire.
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How it works
First you start with your image that's been scanned as a positive. You need to crop out the frame borders and clone out any dust or scratches first. It should look something like this.
Then an invert adjustment layer is added and you get a positive image with a heavy cyan cast.
Then the first curves adjustment layer is applied, which does 90% of the magic. It essentially removes the inverted orange mask from the film base by cutting off the ends of each color channel which contain no information (see the curves dialog in this screenshot). I should note that there is no clipping of color information performed during this step, which preserves maximum image quality. Many tutorials I've followed describe performing this step manually by moving the curves clipping points while holding down the Alt key so that you can stop and pull back once you see the color channel begin to clip. That works, but why not automate it with way more reliability?
When that's done, we're left with a mostly color corrected image, but it lacks contrast and there might still be a faint color cast that remains.
The next step applies a second curves adjustment layer on top of the first that clips the color information ever so slightly, specifically only 1% in the shadows and 0.2% in the highlights. This last step increases contrast slightly while also reliably removing any remaining color casts from the image (most of the time). What you are left with is an image that will require very little further edits, if any.
That's it, you're done! Below are a few more examples using this process.