Pan and scan / Reframe
Watch as an animation →When you change a wide picture to a narrow ratio like vertical or square, move the crop window toward the subject so the important parts stay in the frame.
When re-releasing a 16:9 ad as 9:16 Shorts or Reels, or when you have no budget to regenerate for each ratio.
If possible, it is better to generate separately for each ratio (see Aspect Ratio). If you have to crop, look at where the subject is in each cut and set the crop window. In ffmpeg, crop with crop=ih*9/16:ih:x=<X>:y=0,scale=1080:1920, and if the subject moves, put a time expression in x like '(iw-ow)*t/4' to follow it slowly. Enlarging the 608×1080 cropped from 1920×1080 to 1080×1920 makes it blurry, so make up for it with upscaling if needed (see Upscaling). Premiere Pro's Auto Reframe and Resolve Studio's Smart Reframe follow the subject automatically, but they easily miss people and text at the edge of the frame, so check every cut.
Reframe a 1920x1080 clip to vertical 9:16 with the crop window drifting slowly from left to right over 4 seconds to follow the subject: ffmpeg -i wide.mp4 -vf 'crop=ih*9/16:ih:x=(iw-ow)*t/4:y=0,scale=1080:1920' -c:a copy out.mp4. For a still subject, replace x with a fixed pixel position.
Not verified