darktable preset inspired by FujifilmNeopan 100 Acros II

What the style changes

The style converts the photo to black and white the way Acros II sees color, then adds its clean highlights, readable shadows and very fine grain. What it is based on is detailed on the Fujifilm Acros 100 II 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 Fujifilm’s published spectrogram. The curve is integrated against the CIE 1931 color matching functions, after dividing out the 5400 K daylight it was measured under: this is how darktable builds its own Ilford presets. Its blue weight (Z 0.24) is far lower than in the Tri-X 400 (0.39), Fomapan 100 (0.40) and HP5 Plus (0.49) styles.
  • Compared with a plain luminance conversion, and before any curve or grain, the sky gets only a little lighter (L 55.2 → 57.0, against 58.4 with Tri-X and 59.3 with HP5 Plus weights). A blue box goes from 48.5 to 51.7. Skin loses 1.5 L* (3.8 with Tri-X) and a pink T-shirt gains 0.6.
  • With the full style, the sky reads 58.6 and the blue box 52.2, against 60.7 and 55.2 with the Tri-X style. On a denser sky (L about 42), both styles land at 42.1: the Tri-X curve darkens the lower tones more, which cancels its extra blue weight.
• Moderate contrast. Contrast rises from 14.3 to 17.0 on one photo and from 30.7 to 33.3 on another: between the base render and the Tri-X style (18.0), a compromise between the reviews that find the film soft and those that find it punchy.
• Rich shadows that stay readable. The darkest 5 % end at L 1.4, against 0.8 with the Tri-X style and 0.3 with the Fomapan style. A dark vault keeps 2.5, against 1.3 and 0.5.
• Clean whites. The brightest 5 % reach 91.6 and a white wall 91.4, the highest of the three styles, with no visible clipping.
• Very fine grain. Measured on a sky at 2560 px, the grain comes out at 2.4, against 4.9 for the Fomapan 100 style and 6.8 for the Tri-X 400 style: about the ratio of the RMS figures (7 against 13.5). At 1000 px it almost disappears.
• 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.
• Reading the chart. The spectrogram was digitized pixel by pixel, to about ±0.02 log. Past 662 nm, where the printed curve ends, sensitivity is counted as zero.
• The light source. The 5400 K daylight of the datasheet is modeled as a black body. Between 5000 and 5800 K, the Z weight moves from 0.255 to 0.224, always well below Tri-X (0.386).
• It does not use darktable’s Acros preset. The built-in “monochrome | Fuji Acros 100” preset (0.333, 0.313, 0.353) comes from a 2013 blog post that does not describe its calculation, and targets the original Acros. Our reading of the original Acros datasheet gives (0.357, 0.354, 0.289); the preset lightens skies more.
• Orthopanchromatic or panchromatic? Fujifilm and some reviews use different words. The style follows the published curve, not the word.
• Modest sky darkening. On a dense sky it disappears behind the effect of the tone curve. On film, dark skies still call for a filter: Fujifilm gives daylight factors of 2 for yellow, 4 for orange and 8 for red.
• Contrast. The reviews disagree. The style aims at the datasheet’s nominal contrast in D-76; another developer or push processing changes the result a lot. The rising gradient of the datasheet is only partly reproduced, because display white is capped at 1.
• The grain. darktable’s grain is procedural noise: it imitates the fineness of the grain, not the shape of Acros’s crystals.

Tips

• For darker skies, lower the Z (blue) weight of the “N&B” instance: it acts like a yellow or orange filter.
• See the spectral conversion alone by switching off the style’s tone curve and grain instances.
• Back to color: switch the main color calibration back on and switch off the “N&B” instance.
• Installation: see Install and apply darktable styles.