How to crop a video
Drop in a video and it opens with a square crop frame over the picture. Pick 1:1, 9:16, 16:9 or 4:5 under Crop, then drag the frame to choose which part of the picture stays; drag its corner to make it smaller and zoom in. Arrow keys move a focused frame in small steps. Original turns the crop off.
Which shape for where
9:16 fills a phone screen for TikTok, Reels and Shorts. 1:1 and 4:5 suit an Instagram feed, where 4:5 takes up the most room. 16:9 is YouTube and a TV. A crop keeps only pixels that were there: a 1920 by 1080 video cropped to 9:16 comes out 606 by 1080, the full height and a slice of the width.
The frame stays put
The crop is in one place for the whole clip; it does not follow someone who walks across. Scrub through with the playhead before saving and place the frame where the subject stays in view, or trim the clip to the part where it does.
Questions
Can I crop to another shape, like 21:9?
No, only 1:1, 9:16, 16:9 and 4:5, or no crop at all. The frame can be made smaller to zoom in, but it keeps its shape.
Will the cropped video be lower resolution?
It is smaller by what was cut away, and never enlarged to make up for it: Size only offers sizes smaller than the crop.
Can I add black bars instead of cropping?
Not to a video. For a picture, the Social Media Image Resizer can pad it out to a platform's shape.