lightcraft/docs/fuji-lens-corrections.md

Fujifilm embedded lens corrections

LightCraft reads the distortion and vignetting tables that Fujifilm cameras store in the raw IFD of every RAF (0xf00b GeometricDistortionParams, 0xf010 VignettingParams, signed rationals) and turns them into the existing OpcodeList3 pair, WarpRectilinear and FixVignetteRadial. The profile-correction control, the CPU / GPU optics path, EXIF orientation handling and DNG export use them like any other embedded correction.

Corrected: distortion (with the camera's frame-filling zoom) and vignetting, for the 59 bodies listed in crates/raw/src/vendor/raf_lens.rs (VALIDATED); the nine-knot bodies among them other than the X-T4 were checked with an earlier reading of their tables than the one they now get (see "Knot radius"). Not corrected: lateral chromatic aberration (the third table, 0xf00f); see "Chromatic aberration" below. Bodies without a checked sample, and tables in a layout or normalisation that was not understood, stay uncorrected.

Clean-room evidence

The tag names come from the ExifTool FujiFilm tag-name documentation; ExifTool was only run as a black box (exiftool -j -n, -v3) to print the arrays. Everything else was measured: the 129 CC0 RAFs with these tables under raw.pixls.us (Fujifilm, 65 bodies) against the camera's own embedded JPEG, which the cameras correct in-camera, and against LightCraft's uncorrected raw render. No other decoder's code or data, lens database or Adobe profile was consulted. Scripts and per-file results: data/testing/raf-lens/ (local).

Layout and units

Every table is an array of signed rationals with n knots:

[scale, knot_1 … knot_n, value_1 … value_n]

• scale × n is the half diagonal of the recorded image (the RAF crop) in pixels: 3749 for 6240×4160, 4643 for 7728×5152, 2942 for 4896×3264, 7280 for 11648×8736, on 61 of the 65 bodies. Files that store another number (X-S20, X-T30 III and X-M5 write the 40 MP value, 4643; an X-T5 in an in-camera crop) are left uncorrected.
• Knots are radii in half diagonals: the farthest corner is at 1, and a table is read at ρ = r, the distance from the centre in half diagonals. Older bodies use 11 knots 0, 0.1 … 1, newer ones 9 knots evenly spaced in r² (√(k/8)), GFX 50 bodies 8 (√(k/8), last 1). In the nine-knot tables the eighth knot is the corner and the ninth, √(9/8) = 1.0607, lies beyond it. In all 1,140 tables of 570 X-T4 raws, taken with three lenses at every focal length they were shot at, the ninth repeats the eighth's value. The distortion zero and the 100 % illumination at the axis are anchored at ρ = 0 when the first knot lies above it.
• Distortion values are percent: the bracket 1 + D/100 multiplies the radius.
• Vignetting values are percent of the axial illumination at the (source) radius.
• The chromatic-aberration table has the layout [scale, knots, red × n, blue × n, scale] (fractions of the radius, the displacement of the red and blue planes relative to green).

Geometry

The camera's JPEG samples the raw at

source = z · r · (1 + D(source) / 100)
z      = 1 / max over the output border radii of (1 + D(source) / 100)

The table is indexed by the source (sensor) radius, not the output radius, and the zoom z keeps the output rectangle inside the source (as for the Sony table, the bracket is largest at the nearest border radius). Source radius is solved per radius by fixed-point iteration, then source(r) = r (k0 + k1 r² + k2 r⁴ + k3 r⁶) is fitted (minimax, 1025 samples) for the DNG warp; a fit that strays more than 0.0008 half diagonals (largest over the 118 sample tables 0.0007, median 0.00012) or folds is rejected. The correction itself is the warp's bracket; the camera's rounding and interpolation between knots are not reproduced beyond linear interpolation between knots.

Vignetting is the gain 100 / V(r) at the source radius, fitted by a 5-term minimax polynomial in r² (FixVignetteRadial); a fit more than 0.025 off the table is rejected.

Validation

The tables were first read with the last knot at the corner (ρ = knot_last · r). For the 11-knot and GFX 50 tables, whose last knot is 1, that is the same reading. For the nine-knot tables it reads every value from 6 % further out; "Knot radius" below has the evidence that changed it. The figures in the rest of this section were measured with the earlier reading.

Method: LightCraft's uncorrected render of each RAF (1200 px long edge) against the camera JPEG (camera-jpeg/, resized to the same size). Per file the high-passed luminance is correlated (NCC) after a warp with a free global scale and shift; the warp is (a) similarity only, (b) the table with its sign flipped, (c) the table, lookup at the output radius, (d) the table, lookup at the source radius with the computed zoom (adopted), (e) a free three-term radial polynomial (the upper bound). 67 body/lens/focal-length combinations from 65 bodies, 26 named lenses and the fixed-lens cameras (129 files).

Warp	median NCC	mean NCC
similarity only (nothing corrected)	0.941	0.865
table, opposite sign	0.493	0.530
table, lookup at the output radius	0.989	0.869
table, lookup at the source radius, computed zoom (adopted)	0.991	0.872
free radial polynomial	0.994	0.985

The sign is determined (the 48 combinations with more than 1 % distortion reach a median of 0.29, at most 0.91, with the wrong sign), the source-radius lookup is slightly better where distortion is large (GFX100RF 0.985 vs 0.971, X-S20 0.989 vs 0.971, X-S1 0.977 vs 0.961). The table reaches the free fit's correlation within 0.02 for 51 of 67 combinations including the computed zoom, and for 60 of 67 when the zoom is free (shape right). The zoom z agrees with the free fit's scale within 0.0047 on all 51.

Where the framing differs: on five corrected combinations the camera did not zoom (XC16–50 at 16 mm on the X-A10, XC15–45 on the X-A5 and X-T100, the F770EXR and HS50EXR) and on two more it zoomed about 1 % differently (X-T30 III, XQ2): the shape matches but the JPEG shows 4 % more of the sensor than the computed zoom does. The same body and table zoom on other lenses (the X-A1 and X-A2 with the XC16–50), so the rule is not in the table. LightCraft uses the computed zoom everywhere (no empty border, as for the Sony table). Six bodies whose shape did not match well enough (F550EXR, S1, SL1000, X10, XF1, XQ1) are not corrected.

Through the shipped code path (lightcraft-cli render of the same 67 combinations with the embedded correction applied, then the free fit of the corrected render against the camera JPEG, global scale removed so the framing difference does not count): the residual radial shape error falls from a median of 0.72 % of the half diagonal (largest 2.60 %) to a median of 0.05 % (largest 0.42 %), 54 of the 57 corrected combinations land within 0.25 % (2 px at 1440 px), and the similarity-only correlation rises from a median of 0.957 to 0.993. Per-lens rows are in data/testing/raf-lens/out/after_table.md.

Vignetting

Brightness of the camera JPEG against the aligned uncorrected render, in linear light, modelled per file as log Y_jpeg = poly3(log Y_render) + β(radius bin) (the polynomial absorbs the tone curve): the JPEG's gain β against -ln(V/100) of the table over 42 combinations whose corner gain exceeds 5 %: correlation median 0.987 (37 of 42 above 0.9), proportionality factor 0.98 (interquartile 0.73–1.31), against the tone curve's own slope of about 1.05; flat tables (X-M5, X-T2, X-T3) give a gain of zero in the JPEG. Through the shipped code path the mean |β| over the outer three radius bins of the corrected render falls from a median of 0.184 to 0.031 (log units; largest 0.81 to 0.15) on the 38 corrected combinations with a corner gain above 5 %, and the corner β at least halves on 30 of them. Seven combinations show only a fraction of the amplitude (S1, SL1000, X-A2, X-A3, X-S1, X-T1, X-Pro3); the camera setting that governs them is not recorded.

Knot radius

On the X-T4, the nine-knot tables read at the plain radius match the camera and the earlier reading does not:

• Geometry, against the free fit measured above (matches_the_measured_x_t4_geometry, pixls 3913, XF16-55 at 16 mm): the camera's scale is 1.0300. Read at the plain radius the table gives a zoom of 1.0298 and stays within 0.0008 half diagonals of the fit at every radius; the earlier reading gave 1.0333 and strayed up to 0.0034
• Geometry and gain, against synthetic charts the camera converted. Copies of the same raw with patches of known colour, converted by the camera in its USB raw conversion mode. Against the camera's geometry from 445 chart markers, the distortion table read at the plain radius is at most 1.2 pixels off at 3120 × 2080 out to 0.92 of the half diagonal (earlier reading 4.2), and against its corner gain from 46,818 flat windows the vignetting table is within 1.2 % out to 0.9 of the half diagonal (earlier reading up to 6.5 % too bright, 5.9 % at 0.8)
• On photographs, 500 of the contributor's own X-T4 raws (XF16-55, XF70-300 and a 13 mm lens), shown only as aggregates. With the earlier reading the table bends inside the frame, where the ninth knot's repeated value begins (1 / 1.0607 = 0.943 of the half diagonal), and on 253 of the 425 zoom-lens raws the warp polynomial failed its fit checks, so they got no distortion correction at all. Read at the plain radius the bend is at the corner and 424 of the 425 get their warp. The 75 raws from the 13 mm lens record an all-zero distortion table, so they have nothing to correct either way. Against the camera's own conversion of each raw (LightCraft's render at 1,200 pixels, compared at 780 × 520), the correlation of the high-passed luminance went from a median of 0.03 to 0.90 on the XF16-55 (327 raws) and from 0.02 to 0.83 on the XF70-300 (91), and stayed at 0.96 on the 13 mm. Over flat areas, LightCraft's corners less its centre, against the camera's, went from +0.56 to +0.21 L* on the XF16-55 and from +0.49 to +0.21 on the XF70-300. A further 292 raws held back from all of this gave the same: correlation 0.06 to 0.91 and 0.21 to 0.85, corners +0.62 to +0.26 and +0.35 to +0.14 L*

The other bodies with nine-knot tables are read at the plain radius too, because their knots have the same layout (√(k/8), the eighth at the corner), but none of them was measured at it: their place in VALIDATED rests on the figures under "Validation" above, taken with the earlier reading, and is unconfirmed for the plain radius until a camera JPEG from each is checked. Render cache version 66 redraws thumbnails rendered with the earlier reading.

Chromatic aberration (not applied)

The CA table is real: on the raw mosaics themselves (uncompressed RAFs, colour planes built by normalised convolution so no demosaic is involved) the radial shift of the red and blue sites against green correlates with the table over 46 curves from 30 files at a median 0.93 (31 above 0.8), with the table's sign, at about 0.7 of its amplitude (the same estimator reads 1.0–1.15 on a synthetic mosaic with the table applied). The camera JPEGs show no measurable red/blue shift, but 4:2:0 chroma shares the luminance edges, so that proves nothing.

LightCraft's own demosaic, however, already registers the colour planes: its render shows red/blue shifts of only 0.05–0.2 px where the table has 1.5–5 px. Applying the table as per-channel warp planes (tried; the opcode supports them) shifted the planes the wrong way: the median measured shift rose from 0.10 to 0.13 px and grew by more than 1.5× on 11 of 28 curves (X-T4 blue 0.16 → 0.58 px, X-T3 red 0.06 → 0.59 px). The table is therefore not applied; a correction before demosaicing would be needed.

Tests

• crates/raw/src/vendor/raf_lens.rs: layout, normalisation, the model list, malformed tables, the zoom and the vignette gain on the X-T4's real arrays, and the X-T4 geometry against its measured free fit.
• crates/raw/src/vendor/raf.rs: a synthetic RAF through header and full decode, only for validated models, and a DNG round trip.
• crates/raw/tests/it/corpus.rs fuji_embedded_lens_tables_follow_the_validated_models: the corpus' raf-fuji-* files, or any folder of RAFs in LIGHTCRAFT_FUJI (the pixls set: 129 decoded, 88 warps, 98 vignette gains).