Should I crop or extend an image to change its aspect ratio?

Use a crop for small changes, such as 1:1 to 4:5, or when the original has generous background. Use an extension for big changes, such as square to 16:9 or wide to 9:16, where a crop would keep too little.

Extensions work best into simple surroundings: sky, wall, floor, sand or studio backdrop. They are riskier next to people, text and products, where the model has to continue something specific. Adobe's Generative Expand, for example, fills an enlarged canvas with new content, with or without a prompt.

You can also combine them: extend a little, then crop to the exact size. Keep the original version, so you can go back if an extension adds something wrong.