Nikon embedded distortion corrections

Nikon writes its own distortion correction into NEFs from its mirrorless bodies, in the raw SubIFD tag 0xc7d5 (NikonNEFInfo). Imago reads it for the NIKON Z 6_2 (Z 6II), the NIKON Z 8 and the NIKON Z f and turns it into an OpcodeList3 WarpRectilinear, so it goes through the same profile correction control, optics path, orientation handling and DNG export as the other embedded corrections. Other bodies are left alone for now even if they have the same tag, because I've only checked files from these three.

This is distortion only. The vignetting block (0x0006) and what looks like lateral CA (0x0007) aren't applied yet. Like the Sony, Fujifilm and Panasonic corrections it's on by default and can be turned off under Lens Corrections.

Where the data comes from

The ExifTool Nikon tag names give the names of NikonNEFInfo and its DistortionInfo entry (0x0005): DistortionCorrection and RadialDistortionCoefficient1 to 3. Nothing else was taken from other software. The rest was worked out by comparing Imago's uncorrected render of each NEF with the full-size JPEG the camera embeds in it. That JPEG is always distortion-corrected with Z lenses ("On (Required)"), and with other lenses when Auto distortion control is on. The comparison matched textured patches between the two images (normalised cross-correlation on the green channel at quarter size).

NikonNEFInfo starts with "Nikon\0" and a 4-byte version, followed by a small TIFF stream. The two entries used here:

The 0x0008 offset is where the in-body stabilization left the sensor. It's zero with a VR lens, since the lens does the stabilizing, and up to about 100 px with the Z lenses. The center of the distortion measured in the JPEG follows it closely, for example (−90.1, +18.6) recorded and (−91.2, +17.5) measured.

From an output pixel to the raw, around the optical center, the mapping is

rho    = r_out / half diagonal of the image area
r_src  = r_out * (1 + k3*rho^2 + k2*rho^4 + k1*rho^6) / (1 + q)

So the stored order goes from the highest power down. Reading them the other way round (k1 with rho^2) doesn't fit the JPEG at all. q really is the zoom: fitting a separate zoom for every file gives 1 / (1 + q) to four decimals (0.9675 fitted and 0.9676 from q on one of them), so nothing is fitted per file. The decoder turns this into DNG terms the same way docs/sony-lens-corrections.md describes for Sony, with the center in active-area coordinates and the coefficients rescaled to the farthest corner of the active area.

How well it matches

I checked it on 83 of my own Z 6II photos with three lenses between 20 and 200 mm, 45 Z 8 photos (below), plus the CC0 Z f sample in the corpus. The numbers are the radial error against the camera JPEG at full size (6048 px wide), out to 0.95 half diagonals:

On the Z 8 (8256 px wide JPEGs) it looks the same. The 70-200 f/2.8G on the FTZ matches within 0.14 to 0.21 px median at 70, 135 and 200 mm. The NIKKOR Z 24-120mm f/4 S is within 0.6 to 1.0 px median from 36 to 120 mm and 1.5 px at 24 to 28 mm, with the same kind of error toward the edges at the wide end as the Z lenses on the Z 6II. A few 14-24mm f/2.8G frames had Auto distortion control off, so their JPEGs aren't corrected and match about the same either way.

I also rendered a few with imago-cli render at default settings and measured those against the camera JPEG. The median error was 0.10 px for the 70-200 at 200 mm and 0.12 px for the Z f sample. The 24-70 was 0.32 px at 70 mm, on a frame with a 92 px sensor shift, and 8.4 px at 24 mm. The 20 mm came out at 1.3 px. On the Z 8 it was 0.14 px for the 70-200 at 200 mm, 0.45 and 0.68 px for the 24-120 at 68 and 120 mm, 1.8 px for the Z 20 mm and 3.7 px for the 24-120 at 24 mm. That checks the center, crop and DNG conversion along with the model itself.

The F-mount lens on the FTZ matches about as well as I can measure. The native Z lenses still have a smooth error toward the edges that doesn't depend on the sensor shift and is the same in every frame at a given focal length: about +22 px at 0.7 to 0.9 half diagonals at 24 mm, +8 px at 33 mm and +3.5 px at 70 mm. It seems the camera applies an extra correction for Z lenses that isn't in the readable coefficients. NikonNEFInfo 0x000e looks encrypted and I didn't try to decode it. Even so, the readable correction takes most of the distortion out. At 24 mm the error at 0.7 half diagonals goes from −82 px to +25 px, and at 70 mm from about −137 px to under 4 px.

What's left out

Only NIKON Z 6_2, NIKON Z 8 and NIKON Z f are corrected, matched exactly. A matching tag on another body doesn't tell us its pixel pitch or image area, so other bodies can be added once someone checks their files. Only the full-frame 3:2 image area is corrected, because the stored 21.6 mm radius is the FX half diagonal, so DX and other crops are skipped. The decoder also skips data it doesn't recognise (another layout, radius or term count, or no correction mode), values that are clearly wrong (coefficients past ±1, a zoom past ±0.5, a sensor shift over 5% of the frame) and any mapping that folds back on itself before the farthest corner.

The tests in nef_lens cover the polynomial and zoom, the sensor-shift center and refusing other bodies, layouts and crops. nikon_in_file_distortion_becomes_a_warp runs a synthetic Z 6II NEF through the header and full decode and a DNG export. nikon_in_file_distortion_is_limited_to_validated_bodies checks the corpus: the Z f sample gets the warp and the D5100, D7000, D7500 and Coolpix B700 samples don't.

None of my photos, crops or reference images are committed, only the numbers above.