A one-command way to convert the heavy fisheye curvature from a 7Artisans 7.5mm Mark II f/2.8 (a APSC diagonal fisheye on MFT) into a natural-looking rectilinear image, using only ffmpeg. No GUI, no plugins, no lens profile required.

The command

ffmpeg -i in.jpg -vf "v360=input=fisheye:output=flat:id_fov=142:d_fov=142:w=9216:h=6912:interp=lanczos,crop=6062:3648:(iw-ow)/2:(ih-oh)/2" out.png

That’s it. Swap in.jpg / out.png for your filenames.

Background

The 7.5mm is sold as a ~190° fisheye, but that figure is the diagonal angle measured in the lens’s own fisheye projection — feed it to a rectilinear remap directly and it badly over-corrects. What you actually want is the effective diagonal field of view of the frame, which for this lens/body lands around 142° once tuned (see Calibrating below).

ffmpeg’s v360 filter reads the frame as a fisheye projection and re-projects it to flat (rectilinear / gnomonic) — the “normal lens” projection in which straight lines stay straight.

The pipeline, step by step

1. Fisheye original (straight out of camera)

Note the bowed rooflines and the curved iron railings.

fisheye original

2. After v360 remap, before cropping

The projection is now rectilinear — lines are straight — but mapping a curved frame onto a flat one leaves empty black wedges at the edges where no pixels exist.

defished, uncropped

3. After the centered crop

The crop filter trims those wedges away, leaving a clean rectangle.

final, cropped

What each parameter does

Parameter Value Meaning
input=fisheye — Treat the source as a fisheye projection.
output=flat — Re-project to rectilinear (straight lines stay straight).
id_fov 142 Input diagonal FOV of the lens, in degrees. The key correctness knob.
d_fov 142 Output diagonal FOV — framing/zoom. Set equal to id_fov to keep all captured content.
w / h 9216 / 6912 Output resolution (here 2× the 4608×3456 source).
interp lanczos Interpolation quality. Default is bilinear (softer).
crop=W:H:X:Y 6062:3648:(iw-ow)/2:(ih-oh)/2 Removes the black wedges. The expressions auto-center the crop.

Correctness vs. framing — keep these separate

  • id_fov decides whether lines come out straight. It describes the lens. Get it wrong and nothing else fixes it. Over-corrected (edges pushed out, stretched) → id_fov too high. Under-corrected (still bowed) → too low.
  • d_fov and crop decide what you see. They don’t affect correctness. Keeping d_fov equal to id_fov preserves the full captured frame, then crop reclaims a rectangle from it.

Removing the black corners

The crop filter runs last in the chain. The expression form auto-centers, so you only set the output rectangle:

crop=6062:3648:(iw-ow)/2:(ih-oh)/2

Here iw/ih are the incoming (post-remap) dimensions and ow/oh are the width/height you set — (iw-ow)/2:(ih-oh)/2 always centers. crop is a pure pixel cut: no resampling, no quality loss. To find your own W:H, render once without the crop, read the largest clean rectangle off the result in any image viewer, and plug those numbers in.

Quality notes

  • interp=lanczos is the cheapest quality win over the bilinear default. spline16 is a slightly softer alternative that rings less on high-contrast edges.
  • Feed it the best source. If you shoot raw, develop to a 16-bit TIFF and run that through ffmpeg rather than a camera JPEG. Out-of-camera JPEGs are a hard ceiling.
  • Don’t re-compress at the end. PNG/TIFF output is lossless. For JPEG, use -q:v 2.

  • Supersampling (render at 2× then downscale with lanczos) further reduces edge aliasing. The w=9216:h=6912 above is already 2× the source; add a trailing scale if you want to deliver at native size:

    ...interp=lanczos,crop=6062:3648:(iw-ow)/2:(ih-oh)/2,scale=3031:1824:flags=lanczos

Calibrating for your setup

Lens sample variation and how you read FOV mean your ideal id_fov may differ slightly.

  1. Shoot something with strong straight lines near the frame edges (a building, a doorway).
  2. Start from id_fov=142 (with d_fov equal to it).
  3. If edges are still bowed, raise id_fov; if over-corrected/stretched, lower it. Adjust in ~1° steps.
  4. Once lines are straight, set crop purely for framing.

Lens: 7Artisans II 7.5mm f/2.8 · Body: Olympus E-M10II (4:3) · Tool: ffmpeg v360 filter. Example image: Dublin Georgian terrace.