darktable preset inspired by KodakPlus-X 125

What the style changes

The style converts the photo to black and white the way Plus-X sees color, then adds a soft toe, moderate contrast and very fine grain. What it is based on is detailed on the Kodak Plus-X 125 film page. The figures below compare the full style with darktable’s base render, measured in CIELab.

• Gray from the film’s spectral sensitivity. A color calibration instance named “N&B” mixes the channels with weights computed from Kodak’s published sensitivity curve. It carries more weight on blue (Z 0.4798) than the Tri-X 400 style (0.3857) and the FP4 Plus style (0.4332). The sky gets lighter (L 55.3 → 60.8), while skin (78.2 → 76.7) and a pink T-shirt (47.3 → 45.1) get darker.
• Moderate contrast, soft toe, open highlights. The tone curve gives a long, soft toe, midtones of medium gradient and no shoulder. Contrast rises from 14.3 to 16.5 on one test photo and from 30.7 to 31.7 on another, less than in the Tri-X style (18.0 and 33.5). The darkest 5 % of the frame drop from L 2.6 to 1.4, less deep than with Tri-X (0.8) and FP4 Plus (1.1). The brightest 5 % go from 75.2 to 78.4 on one photo and from 87.9 to 88.1 on the other.
• Very fine grain. Grain is sized from the RMS granularity of 10. On the sky of one test photo, exported at 2560 px, its measured level (3.5) equals the FP4 Plus style and is about half that of Tri-X (6.8).
• 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. darktable’s built-in black-and-white presets also bypass chromatic adaptation, and film does not white-balance either.

• It also replaces your tone curve and grain. When you apply a style from the lighttable or the darkroom, each of its modules replaces the instance of the same module with the same name, if the photo has one, and is added otherwise. 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.

Exporting with a style, or rendering with darktable-cli, works differently: every module of the style is added as a separate instance, and the disabled color calibration still switches off the existing one. The details are in Install and apply darktable styles.

Limits

• An interpretation, not a calibration. No film scan was used to tune the style.
• The spectral curve. It was first read by eye on a log-scale graph (±0.05 log), and the weights are sensitive to that reading, above all for the violet (peak at 408 nm, cut-off near 380 nm). A later, finer trace of the same graph is shown on the film page.
• The illuminant. The datasheet does not state the illuminant of the curve. The style assumes equal energy, as for Tri-X.
• The developer. Kodak’s characteristic curves are given for D-76 and T-Max, with 5 to 11 minutes. The style aims at an average development (7 min in D-76); real contrast changes with the developer.
• The grain. RMS was measured in HC-110 (B) and is not directly comparable with other manufacturers’ values. darktable’s grain is procedural noise: it imitates the texture of film grain, not the shape of real silver grains.
• The shoulder. The style’s curve acts on an image already rendered by sigmoid, so the absence of a shoulder is approximate.
• Thin review evidence. The reviews are few and sometimes about expired film, and none describes how the film renders colors as grays: the style relies on the datasheet alone.
• Close to FP4 Plus. Both styles have the same RMS and similar curves. The difference comes from the stronger blue weight and a curve without a shoulder. It is small, and it matches sources that describe the two films as close.

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.
• See the spectral conversion alone by switching off the tone curve and grain modules the style set.
• Back to color: switch the main color calibration back on and switch off the “N&B” instance.
• Installation: see Install and apply darktable styles.