darktable preset inspired by IlfordFP4 Plus

What the style changes

The style converts the photo to black and white the way FP4 Plus sees color, then adds its medium contrast, its shoulder and a fine grain. What it is based on is detailed on the Ilford FP4 Plus film page. All figures below come from renders made with darktable-cli 5.6.0 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 Harman’s published spectrogram. The curve is integrated against the CIE 1931 color matching functions, after dividing out the 2850 K tungsten light it was measured under: this is how darktable builds its own film presets, such as “monochrome | ILFORD HP5+”.
  • Compared with a plain luminance conversion, and before any curve or grain, the sky gets lighter (L 55.2 → 58.8), while skin (79.3 → 74.7) and a pink T-shirt (47.8 → 46.0) get darker.
  • Skin darkens less than with HP5 Plus weights (−4.6 against −5.7): FP4 Plus is less sensitive to blue and peaks further toward orange.
• Medium contrast. The tone curve keeps a gradual toe, firms up the midtones a little and bends into a shoulder near white. Contrast rises from 14.3 to 16.9 on one test photo and from 30.7 to 32.1 on another, less than with the Tri-X 400 style (18.0 and 33.5).
• Deep blacks, held-back whites. The darkest 5 % of the frame drop from L 2.6 to 1.1, and a dark brick vault keeps its texture. The brightest 5 % reach 88.5, against 90.6 with the Tri-X style: that is the shoulder at work.
• Fine grain. Measured on the same sky at 2560 px, the grain comes out at 3.5, against 4.8 for the Fomapan 100 style and 6.8 for the Tri-X 400 style. These values roughly follow the ratio of the three films’ RMS figures (10, 13.5 and 17).
• 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. We measured a blue cast of b* −4 to −8 in our renders before adding the disabled instance. 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 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. Our measurements were made that way, on darktable’s default rendering. The details are in Install and apply darktable styles.

Limits

• An interpretation, not a calibration. No film was scanned or measured to tune the style.
• It does not use darktable’s FP4+ preset. darktable 5.6 ships a “monochrome | ILFORD FP4+” preset (0.2415, 0.2215, 0.5370) that cites the same datasheet, but its weights are close to what a completely flat sensitivity would give (0.237, 0.224, 0.540 by our calculation), so we could not match it to the curve. With the preset, a sky comes out about 1 L* lighter and skin about 2 L* darker than with the style’s weights.
• The spectral axis. Harman’s chart has no unit. Like darktable, we read it as log sensitivity, a reading checked on HP5 Plus. If the curve spanned 0.8 or 1.2 log units instead of 1, the X weight would move between 0.277 and 0.302 and the Z weight between 0.456 and 0.410.
• One developer. The datasheet gives the characteristic curve in Ilfotec HC only, and reviews say FP4 Plus changes a lot with the developer. The style aims at a normal development.
• The grain figure. The RMS of 10 comes from the motion-picture version in Kodak D-96. It is only roughly comparable with Foma’s value (Microphen) and Kodak’s (HC-110 for Tri-X).
• The shoulder. On film it starts about 7 stops above the shadows and touches only bright highlights. The style’s shoulder acts on an image already rendered by your tone mapper, so it is an approximation.
• The grain. darktable’s grain is procedural noise: it imitates the texture of film grain, not the shape of real silver grains.

Tips

• See the spectral conversion alone by switching off the style’s tone curve and grain instances.
• Prefer darktable’s own weights? Apply the built-in “monochrome | ILFORD FP4+” preset to the “N&B” instance: it only changes the channel weights.
• Back to color: switch the main color calibration back on and switch off the “N&B” instance.
• Installation: see Install and apply darktable styles.