Color Neg Resources

Luts and scripts for processing color negative film scans in photoshop.

These files can be used with the process described in my post Scanning Color Negative Film

A TLDR

Details on the process are in the post linked above.

To process a color negative scan you will need:

• A linear camera profile
• A linear color profile to use as a working space.
  • I recommend ACEScg-elle-V4-g10 from elles_icc_profiles

The main steps to the process are:

• White balance
  • Compensate for: backlight, film base color, and lighting of individual film frames.
  • Works by multiplying linear raw rgb values.
• Density balance
  • Compensate for differences in measured dye density between the channels.
  • Works by multiplying density, or a power function on the linear values.
• Invert
  • Invert colors by dividing
  • positive_value = constant / negative_value

Density balance script

density balance.js is a photoshop script that calculates density balance and automates the creation of adjustment layers from two gray color samples. It can be used with a film photo of a color checker.

To use the script and create an action:

• Use the eye dropper tool with sample size set to an average
• Set the foreground color to the darker gray patch, lower density (light) negative
• Set the background color to the lighter gray patch, higher density (dark) negative
• (to set up) Start recording a new action
• Run density balance.js script with: File > Scripts > Browse
• Create a Color Lookup adjustment layer with inverse_01.cube
• Stop action

Negative to positive 3d lut

The process can be baked into a 3d lut that can be used directly in acr/lightroom.

The python script neglut.py will generate a 3d lut cube file that will:

• convert prophoto rgb to linear acescg
• apply density balance and invert
• apply a tone curve (optional)
• convert colors back to prophoto

The lut is limited in what it can do, but can be very useful for quickly getting a finished image or flat positive for editing. The only adjustments that can be used are Exposure and White Balance. Changing any other setting would disrupt the linearity of the negative.

To use the script

After you have decided on values to use for density balance in the ACEScg color space, Open the python file in a text editor.

• set the values of redScale, greenScale, and blueScale to your density balance values.
• change savename to the film name or filename you want to use.
• set paper to True or False, for whether or not you want to use the paper curve.

# values for density balance in ACEScg
redScale = 1.0
greenScale = 0.807
blueScale = 0.579

savename = "ektar" # film name

paper = False # True / False: use paper lut

Run the script and it will write the .cube file.

To make a profile in Adobe Camera Raw or Lightroom:

• set white balance to As Shot
• select the linear camera profile
• option/alt click the new preset button
• change the profile name
• select the Color Lookup Table checkbox
• choose the new cube file
• set Space to ProPhoto RGB
• set Amount Min and Max to 100

LUTs

file	lut	function	input	output	input range
inverse_01.cube	inverse	$\frac{0.01}{x}$	linear(negative)	linear	0-1
density.cube	transmittance to density	$\log_{10}(\frac{1}{x})$	linear(negative)	density	0-1
10^x.01_4.cube	density to luminance	$10^{x}\cdot 0.01$	density	linear	0-4
inv_density.cube	inverse density	$\frac{1}{10^{x}}$	density	linear(negative)	0-4
paper_a.cube	paper print curve		density	linear	0-3
paper_invert.cube	paper print curve		linear(negative)	linear	0-1

Paper Print LUTs

A lut to simulate the response curve of photographic paper.
Use an exposure adjustment layer before this lut to adjust exposure and contrast.

paper_a: use in place of 'density to luminance'. offset controls exposure, exposure controls contrast
paper_invert: use in place of 'inverse'. exposure controls exposure, gamma controls contrast