lightcraft/docs/sony-lens-corrections.md

Sony embedded distortion corrections

LightCraft reads the signed 16-sample distortion table in the raw image IFD (0x7037) of Sony ILCE-7RM4A (A7R IVA) Bayer ARWs. Other camera models are deliberately left uncorrected, even when they contain a table with the same layout. It converts that table into OpcodeList3 / WarpRectilinear, so the existing profile correction control, CPU/GPU optics path, EXIF orientation handling, and DNG export use the same correction.

This adds distortion only. Sony vignetting and lateral chromatic-aberration tables are not decoded. The enabled and independently validated model is ILCE-7RM4A with the FE 24–105mm F4 G OSS and FE 200–600mm F5.6–6.3 G OSS. Linear YCbCr ARWs and non-3:2 camera crops are excluded pending validation. An aspect crop may retain the full-frame radial normalization; the decoder does not guess that relationship. Older files that carry only encrypted correction metadata, different table lengths, and rejected tables remain uncorrected. The camera's distortion Off setting does not erase its table. Newly imported photos use LightCraft's existing embedded-profile default (enabled); the profile correction control can disable or adjust it.

Clean-room evidence

The ExifTool EXIF tag-name documentation identifies 0x7037 as 17 signed 16-bit integers. The first value is the count, 16, followed by radial samples. No external photo decoder implementation or Adobe lens profile was consulted. Sony Imaging Edge Edit 4.1 was used only as a black-box reference on local scratch copies.

The observed inverse mapping, with radius r normalized to the default crop's half diagonal, is:

f(r) = 1 + interpolate(table, r) / 16384
source = center + (output - center) * zoom * f(r)
zoom = 1 / max(f(r)) over radii on the rectangular output boundary

The border radii range from the shorter half edge / half diagonal to 1. Table knots span 0 through 1. Linear interpolation approximates the observed map closely; we do not claim to reproduce Sony's internal interpolator. A fixed-size weighted least-squares fit expresses source radius as r * (k0 + k1*r² + k2*r⁴ + k3*r⁶). The fit is rejected if radial error exceeds 0.0005 of the half diagonal (about 0.43 px at a 1440 px 3:2 preview), or the mapping folds. Table values are bounded and the input must have precisely the supported signed-short layout. Crop offset and normalization are converted into DNG's active-area coordinate system before creating the opcode. The warp's centre and normalization come from the standard DefaultCropOrigin / DefaultCropSize when the file has them, not from Sony's crop tags: on the tested A7R IVA files the former start at raw x=32 and the latter at x=0, and the standard origin is the one that aligns with Sony's exports. The image itself keeps the decoder's crop precedence for every Sony body (Sony's crop tags, then the default crop, then the recorded image size), so the framing is unchanged; the opcode's centre is in active-area coordinates, so the warp stays on the optical centre either way. On the A7R IVA that framing still sits 32 raw pixels left of Sony's exports (eight pixels in a 2376-pixel-wide export).

The main raw-IFD table is important: the similarly named MakerNote table has different values and must not be substituted into this formula. Main raw-IFD and SR2SubIFD values agreed in the test files.

Validation

At 2376×1584, independent SIFT correspondences between Sony distortion On and Off TIFF exports gave:

Lens / focal length	Median residual	95th percentile	Maximum matched radius
FE 200–600 / 200 mm	0.043 px	0.191 px	0.97 half diagonals
FE 200–600 / 600 mm	0.055 px	0.231 px	0.95 half diagonals

These use the fixed 16384 scale and automatic border framing, not fitted per-image scale or zoom. A median translation below 0.004 px was removed from the residuals. Correspondences do not cover all pixels; these are geometric feature residuals, not photometric equality or a worst-case guarantee. Across 20 sampled RAWs (16 at 24–105 mm, four at 200–600 mm), the polynomial's maximum deviation from the interpolated file table was below 0.12 px at 1440 px. That measures approximation error, not agreement with an independent renderer. Camera JPEG and Apple Core Image comparisons support the wide-lens interpretation but are weaker evidence than same-renderer On/Off pairs.

Actual LightCraft exports after crop normalization were also compared directly with Sony's corrected TIFFs. At 2376 px wide, median / 95th-percentile feature residuals were 0.35 / 1.04 px at 24 mm, 0.15 / 0.40 px at 200 mm, and 0.13 / 0.56 px at 600 mm (median translation below 0.024 px). The uncorrected medians at 200 and 600 mm were 8.63 and 10.82 px respectively. Different colour, sharpening, and demosaicing affect feature localization; these are geometry checks, not colour parity. An isolated desktop run on Apple M1 Metal detected the profile, rendered without GPU fallback or warnings, and exposed the existing Optics controls. Existing library edits are not reset: once the source has loaded, enable lens corrections if they were previously disabled.

Tests cover signedness/length rejection, zero and malformed tables, independent 600 mm reference geometry, barrel framing, offset crop normalization, both TIFF byte orders, header/full consistency, and preservation through DNG export. No private photographs or reference TIFFs are committed.

Camera coverage and regression boundary

The model check is exact: ILCE-7RM4 (A7R IV without the A suffix), ILCE-7M3, unknown/missing model names, and every other model remain unsupported by this distortion decoder. Matching tag numbers and table lengths do not establish the scale, interpolation or crop normalization on another body. Support can expand after independent geometry validation; do not add model-name prefix matching.

The original nine Sony samples listed by cargo xtask corpus --download were downloaded from its CC0 URLs. The manifest now also includes CC0 samples 4822 (A7R IVA / 24–105 mm) and 3989 (A9 II / 200–600 mm), with their SHA-256 hashes recorded alongside the URLs. All eleven files are checked with sony_embedded_distortion_is_limited_to_validated_models:

Camera	Corpus samples	Raw-IFD table observed	Expected correction
ILCE-7RM4A	Compressed (4822)	16 samples	One distortion warp
ILCE-9M2	Compressed (3989)	16 samples	None
ILCE-7M3	Compressed, uncompressed	16 samples	None
ILCE-7M4	14-bit, lossless L/M/S	11 samples in the 14-bit file; 16 in L/M/S	None
ILCE-7RM2	12-bit uncompressed	16 samples	None
DSC-RX100	One	No raw-IFD table	None
DSC-RX100M3	One	11 samples	None

These are model-boundary regression checks, not new geometric validation of those models. In particular, refusing the A7 III tables resolves the unverified corner warp identified in review. The lossless M/S files are linear YCbCr and remain excluded independently of the model check. Header and full decode must agree for every sample. Synthetic tests also verify both TIFF byte orders, exact model matching, that the warp is centred on the standard default crop while the image keeps Sony's crop, and that DNG export preserves an accepted warp without adding one to rejected models. The existing independent Sony geometry tests continue to cover the enabled A7R IVA path. Re-exporting the 24, 200 and 600 mm reference files after the model restriction produced byte-identical corrected PNGs to the earlier validated exports. Isolated headless app checks showed the A7R IVA profile controls present and the A7 III profile controls absent, with no notices and completed RAW renders.

Before enabling another body, compare corrected and uncorrected renders against independent references across multiple lenses/focal lengths and photographs with features near the edges and corners. Record residuals and spatial coverage, and check crop/aspect and compression variants. Embedded JPEGs are useful when the camera actually applied distortion correction; an Off JPEG cannot certify an On warp. Prefer same-renderer Sony On/Off exports when the camera setting or geometry is ambiguous. Table-fit error alone only checks our polynomial approximation, not whether the table's interpretation is correct.