Back in the days I worked in a photo lab where we developed negative film to color prints. The developing method (C41) was calibrated to a certain film. If you tried to develop film the machine was not calibrated for, your prints ended wrong.
The reason for this was that the negative film consists of three layers of dyes; yellow, cyan and magenta. In ideal situation every layer would only react to its own color, but the dyes are impure, failing to absorb light frequencies they should for good color reproduction. To correct this the film has an orange (yellow, magenta) layer which was corrected during the development process.
If you just invert the picture in Photoshop it will end with a strong cyan tint which must be corrected.
|Negative film with heavy cyan color cast after the inversion in Photoshop|
There’s many ways to do the conversion in Photoshop and I’m not tell if any of them are correct. What’s common for all my methods are that while scanning, you must get a small section of the unexposed part of the film into the scan, it’ll be for great help.
The old analog way to remove the color cast was based on analog color filters. This is something that would logically be the most “correct” method and it is also easy to accomplish with Photoshop.
1. Convert your image to CMYK (Edit -> Convert to Profile -> CMYK)
2. Use the color sampler tool and pick an unexposed part of the film and observe the percentage values of magenta and yellow
|The marker shows an unexposed part of the film|
3. Open the color mixer and reduce the constant of yellow and magenta according to the values of color sampler tool so that they remain 0%
4. Flatten image
5. Convert back to RGB color space and invert the image
However, while this should be a sound method, I have never succeeded without extra tweaking of levels and curves to get it right.
Good proven method
This is a method that yields the best results in my opinion. I’m going to adjust the black and white points individually for every color (RGB) channel separately.
1. Invert the image
2. Open level adjustment layer
3. In the levels tool adjust the gray point from a unexposed part of the image
4. Move the black and white point adjustment sliders individually for every color channel approximately to the point where the histogram begins. More precisely you can hold the alt-key pressed and stop where the pixels start to show. Don’t overdo it, otherwise the great dynamic range of the film will go to waste.
You can also invert negatives in the Lightroom, even if it´s not mentioned for that and the results may vary.
1. Open the tone(point) curve and individual RGB curves
2. Flip all the color curves by grabbing them from the end of the curve so that they will start from the upper left corner.
3. By holding from the end of the curve drag each curve to the point where the histogram begins/stops