Defishing
Also known as: defish, fisheye removal
Straightening the curved geometry an ultra wide lens produces, so lines that should be straight stop bending across the frame.
Updated
What it means
Defishing remaps the image to undo the projection the lens imposed. An action camera bends straight lines near the edges, and the correction pushes them back, trading some field of view for a geometry that looks conventional.
It is the same operation as Lens distortion correction applied to a much stronger lens, and the stronger the original distortion the more the correction has to throw away at the edges.
Cameras offer it as a mode and editors offer it as a filter. The in camera version is baked in and cannot be reversed, which is the practical difference between the two routes.
How it works in practice
Shooting wide and defishing afterwards keeps the choice open. A Linear lens mode in camera makes the same correction permanent, which is convenient and removes the option of changing your mind.
The amount applied is a judgement rather than a setting. Full correction looks geometrically correct and slightly dull; partial correction keeps some of the energy a very wide lens gives action footage.
Checking the horizon after correction is worth doing, because remapping changes how a slightly tilted frame reads and can make a level horizon look bowed. Horizon Lock applied before correction compounds the effect.
What to watch out for
The correction always loses pixels at the edges, so a defished clip is a crop as well as a remap, and the resolution available afterwards is lower than the file suggests.
Stretching near the corners is the visible artefact. Faces and wheels at the edge of a strongly corrected frame look elongated, which is considerably more distracting than the gentle curve it replaced.
Applying it clip by clip also breaks consistency. A sequence mixing corrected and uncorrected footage reads as two cameras, which no amount of Color matching will reconcile.
How RawClip handles it
Footage is analysed and cut with its original geometry rather than straightened first, because correction crops the frame and changes the look. Where a camera baked the correction in, the clip is simply treated as the narrower shot it has become, with no attempt to reverse something that cannot be reversed.