darktable preset inspired by FomaFomapan 400 Action

What the style changes

The style converts the photo to black and white with weights taken from Foma’s spectral sensitivity chart, then adds a contrasty tone curve and visible grain. What it is based on is detailed on the Fomapan 400 Action film page. All figures below come from renders made with tools/validate.py at 1000 px, measured in CIELab.

• Gray from the film’s spectral sensitivity. A color calibration instance named “N&B” mixes the channels with weights computed from the curve on Foma’s datasheet, integrated against the CIE 1931 color matching functions. The chart names no light source and has no vertical scale, so the weights assume 2850 K tungsten and a curve spanning about one log unit, by analogy with the Fomapan 100 sheet.
• Contrast with a tone curve. The curve keeps the blacks slightly lifted, steepens the midtones and lets the highlights rise without a soft shoulder. Contrast (Lsd) rises from 14.3 to 19.1 on one test photo and from 30.7 to 33.3 on another. The darkest 5 % of the frame stay at L 2.7 (2.6 before), and the brightest 5 % rise from L 87.9 to 91.6.
  • This is the intended difference from the Fomapan 100 style, where the same measurement gives L5 0.3 (crushed blacks) and L95 89.2 (softer highlights); Tri-X gives 0.8 and 90.6.
• Visible grain. Grain is sized from the RMS granularity of 17.5. Its strength (34) is slightly higher than in the Tri-X 400 style (32).
• A truly neutral gray. Chroma falls from 8.6 to 0.0: no color cast is left.

What it leaves alone

• Your base development. The style contains no exposure, no sigmoid or filmic and no LUT. Those settings stay as you made them.

• But it switches off your main color calibration. The style contains two color calibration instances:

  • a disabled one with no instance name, which replaces your main color calibration and switches it off. It is set to “as shot in camera” with CAT16, like darktable’s default, so you can switch it back on;
  • the “N&B” instance, which is added and does the black-and-white conversion.

  Why: when an active color calibration runs after the black-and-white conversion, its chromatic adaptation tints the gray.

• It also replaces your tone curve and grain. The style’s tone curve and grain carry no instance name, so they replace any settings you had already made in those two modules. Your earlier settings remain in the history.

• Your pipeline order. The style file carries no module order list, so it does not impose one on your image.

The details of applying a style are in Install and apply darktable styles.

Limits

• Light source and scale of the spectral curve unknown. With equal-energy light instead of 2850 K, the weights would become roughly (0.42, 0.39, 0.19): a much darker sky and lighter skin. The 2850 K choice follows the Fomapan 100 sheet, without confirmation from Foma. With it, the color response is almost identical to the Fomapan 100 style: the difference between the two comes from the tone curve and the grain, not the color.
• An interpreted tone curve. It follows qualitative reviews, not the characteristic curve on the datasheet, which varies with development and exposure.
• Halation and softness not simulated. Halation and the loss of sharpness of 35 mm film on a gray base are not reproduced, because reviews disagree about them.
• The grain. Reviews disagree on its size; the setting follows the datasheet RMS. darktable’s grain is procedural noise, not real silver grains.
• An interpretation, not a calibration. No film was scanned or measured to tune the style.

Tips

• Apply it early. If you plan to fine-tune the tone curve or grain yourself, apply the style first: it would replace those settings if you made them before.
• Back to color: switch the main color calibration back on and switch off the “N&B” instance.
• Installation: see Install and apply darktable styles.