21:9
Also known as: ultrawide, cinemascope ratio
The ultrawide cinema aspect ratio, normally reached by cropping a widescreen frame top and bottom to produce a more filmic shape.
Updated
What it means
21:9 is wider than the 16:9 almost everything records in. The result reads as cinema because the shape itself is a cinema convention rather than a video one. Nothing else about the file changes.
It is normally a crop rather than a capture. A Letterbox applied to a widescreen frame produces the shape and throws away the top and bottom of the image.
True anamorphic capture is the alternative. An Anamorphic lens records the wide field onto a standard sensor and keeps the pixels, which is the difference between the shape and the format.
How it works in practice
Framing has to allow for it. A subject with headroom in 16:9 loses that headroom entirely, so the crop is decided before filming rather than after. Nothing recovers lost headroom.
Resolution absorbs it when there is enough. Cropping 4K to this shape still delivers more than 1080p height, where the same crop from a 2.7K video source is tighter than expected. Height is what runs out.
Horizon discipline becomes strict. A wide short frame exaggerates any tilt, so Horizon leveling in post matters more here than it does in a taller shape.
What to watch out for
It is frequently used as a shortcut to a look. Bars on an unstabilised handheld clip change the shape and nothing else, which is a Cinematic look mistake rather than a format one.
Platforms letterbox it again. A 21:9 video inside a 16:9 player gets black bars, and on a vertical feed the result is a thin strip nobody can see.
Mixed shapes in one edit read as an error. A clip at 16:9 cut against a cropped one looks like a mistake, so the shape belongs to the whole sequence rather than to a handful of shots inside it.
How RawClip handles it
Delivery is a standard widescreen shape, because that is what plays predictably everywhere a highlight gets shared. An ultrawide version is a crop applied to the assembled sequence, which is a request to make rather than a default to accept.