Lens Correction not fixing Vignetting

Matthias1102 · August 7, 2023, 5:10pm

Hello Todd, have you created a bug report at github on this issue?

Or has anyone else created a bug report? Otherwise, I would create one.

I see the same issue for the images created with my Nikon Z6ii and my Nikkor Z 24-120 f/4. My camera and my lens use the latest Nikon firmware.

The issue occurs in darktable 4.4.0 and 4.4.2, since the “image information” on the left side of the darkroom view reports a focus distance of 0.01. In consequence, the lens correction does not apply the vignetting compensation.

This was different in darktable 4.2.1. With this version, the “image information” reports a focus distance of “-“. With this “undefined” focus distance, the lens correction works as expected.

The source code https://github.com/darktable-org/darktable/blob/darktable-4.2.x/src/common/exif.cc#L1036 shows that darktable 4.2 did not read the Exif.NikonLd4.FocusDistance tag at all. This resulted in a better behavior than the current approach https://github.com/darktable-org/darktable/blob/master/src/common/exif.cc#L1187, which seems to read the wrong tag if a Nikkor Z-mount lens is used (w/o FTZ adapter).

Shall I create a bug report at github? Or does it already exist?

tburt · August 7, 2023, 6:23pm

Hey Matthias,

I have not created a bug report. Once I learned that I could manually assign a distance value in the Lens Correction module I’ve been doing so as a work-around. For me the issue appeared after upgrading from 4.4.1 to 4.4.2.

Matthias1102 · August 7, 2023, 6:47pm

Hi Todd,

yes, I have seen that it’s possible to manually assign a distance value in the Lens Correction module. But I have noticed that all of my existing images, which I have processed with darktable 4.2 and before, lose their vignetting correction if I process them with darktable 4.4. This means that I have to manually open all of my old images with darktable 4.4 and adjust the value.

Since this issue seems to occur for all Nikon Z cameras (we already have the Z5, Z50, and Z6ii in this discussion), it would make sense to have an appropriate fix in the exif.cc module.

Therefore, I will create bug report.

Thank you for your immediate response!

Matthias1102 · August 7, 2023, 8:13pm

This is the link to the issue at GitHub:

github.com/darktable-org/darktable

exif.cc reports invalid focus distance for images from Nikon Z cameras with native Z-mount lenses

opened 08:06PM - 07 Aug 23 UTC

Matthias1102

### Describe the bug darktable 4.4.x reports for images from a Nikon Z6ii (late…st firmware) with Nikkor Z 24-120 f/4 (latest firmware) that the focal distance is always 0.01 m. This incorrect focal distance value results in a failure of the vignetting compensation in the Lens Correction module, since this function depends on the focal distance value. The discussion “[Lens Correction not fixing Vignetting](https://discuss.pixls.us/t/lens-correction-not-fixing-vignetting/38363)” provides additional information on this issue: - **The issue seems to occur for all Nikon Z cameras with native Z-mount lenses.** - NEF files from these cameras seem to provide two tags with the name `Exif.NikonLd4.FocusDistance`. One tag seems to refer to F-mount lenses with FTZ adapter, the other tag seems to refer to Z-mount lenses. The latest version of the exif.cc module does not consider the second tag. In darktable 4.2, the issue did not occur, because the old version of [exif.cc ](https://github.com/darktable-org/darktable/blob/darktable-4.2.x/src/common/exif.cc#L1036) did not read the `Exif.NikonLd4.FocusDistance` tag at all. In consequence, the “image information” reported a focus distance of “-“. With this undefined focus distance, the lens correction worked as expected. ### Steps to reproduce 1. Load a RAF/NEF image from a Nikon Z camera with native Z-mount lens into darktable 4.4.x. 2. In the “image information” of the “darkroom” check the “focal distance”. Does it show the true value or 0.01 m? 3. Activate the Lens Correction module. Does it compensate the vignetting of your lens? ### Expected behavior The “image information” should report the true focal distance instead of 0.01 m. The Lens Correction module should compensate the vignetting. ### Logfile | Screenshot | Screencast <img width="908" alt="Screenshot 2023-08-07 214142" src="https://github.com/darktable-org/darktable/assets/43281689/45ba2926-d084-4cec-811e-79ff5936ab90"> ### Commit https://github.com/darktable-org/darktable/commit/ab07d9c5c518137c3fd6f915e145b9c4c321a403 ### Where did you install darktable from? darktable.org ### darktable version 4.4.0 and above ### What OS are you using? Windows ### What is the version of your OS? Windows 11 Pro ### Describe your system? _No response_ ### Are you using OpenCL GPU in darktable? No ### If yes, what is the GPU card and driver? _No response_ ### Please provide additional context if applicable. You can attach files too, but might need to rename to .txt or .zip [example.zip](https://github.com/darktable-org/darktable/files/12284811/example.zip)

kmilos · August 9, 2023, 10:23am

Please help test the proposed fix: exif: fix focus distance parsing for Nikon Z bodies by kmilos · Pull Request #15030 · darktable-org/darktable · GitHub

Would be interesting to check both against exiv2 0.28.0 (e.g. Arch or Tumbleweed) and >= 0.27.4 series.

ctouze · August 12, 2023, 1:40pm

same issue with EndeavourOS exiv2= 0.28.0 and all native Z lenses