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Andrew FrahnandClaude Opus 5.5
80e58ed55b
Raws without an embedded lens correction look their EXIF lens up in the user-supplied database; the profile goes through the existing Enable Profile Corrections path (amount sliders, CPU/GPU geometry stage, DNG export). Synthetic-grid test renders a distorted chart through the pipeline. Sources: own design (see lightcraft-lensdb for the model sources). Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
46 lines
2.6 KiB
Markdown
46 lines
2.6 KiB
Markdown
# Lens profiles from a lens database
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Raws that carry no lens correction of their own (DNG `OpcodeList3`, or a camera correction a raw reader converts,
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see [sony-lens-corrections.md](sony-lens-corrections.md)) can be corrected from a **lens database in the lensfun
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XML format** that you supply. The app does not ship any lens database: lensfun's data is licensed CC-BY-SA 3.0 and
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stays a separate download. Adobe LCP profiles are never used.
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## Setup
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1. Get the lensfun database (the `data/db/*.xml` files of <https://github.com/lensfun/lensfun>, or the copy a Linux
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distribution installs under `/usr/share/lensfun/version_1/`).
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2. Point `LIGHTCRAFT_LENSFUN_DIR` at that directory and start the app (or the CLI).
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Files are read once per process. Invalid files are skipped with a warning in the log. Without the variable nothing
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changes.
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## What is applied
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The lens is identified from EXIF: camera make / model (for the mount and crop factor; the 35 mm equivalent focal
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length is used when the camera isn't listed) and the lens model string. Lens names match on their words; every
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number (focal lengths, apertures) in the database name must appear in the EXIF name, and the lens maker must agree
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when the file names one.
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| Correction | Models | Interpolated over |
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|---|---|---|
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| Distortion | `poly3`, `poly5`, `ptlens` | focal length |
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| Lateral CA (TCA) | `linear`, `poly3` | focal length |
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| Vignetting | `pa` | focal length, aperture (in stops), focus distance (nearest calibrated, by 1/d; infinity when unknown) |
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Fisheye and other non-rectilinear lenses get no distortion correction. The profile is converted by least squares
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into the DNG `WarpRectilinear` / `FixVignetteRadial` polynomials, so it goes through the same path as embedded
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corrections: **Enable Profile Corrections** and its Distortion / Vignetting amounts, the CPU and GPU geometry
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stage (one resample), orientation handling and DNG export. A conversion whose radial error exceeds 0.001 of the
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half diagonal, or that would fold the image, is not applied.
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Normalisation follows lensfun's documentation: distortion and TCA radii are 1 at half the shorter side of the
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calibration frame (its `cropfactor` and `aspect-ratio`), vignetting radii 1 at its corner; the optical centre is
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the image centre (lensfun's `<center>` offsets are not read).
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## Limits
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- Only raws: rendered images (JPEG, HEIF, TIFF) are often already corrected in camera.
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- The focus distance is not read from maker notes yet: vignetting uses the farthest calibrated distance.
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- No manual make / model picker yet; matching is automatic.
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- Not yet validated against real photos and another renderer; the tests use synthetic profiles and grids.
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