The captions you add are also synced across iOS and macOS, so they'll automatically show up on the Photos desktop app on your Mac too.
- Launch the native Photos app on your iOS device.
- Tap a photo in your library that you want to add a caption to.
- Swipe upwards on the photo.
- Tap the Add a Caption field.
- Use the onscreen keyboard to add your caption.
- Tap Done in the top-right corner of the screen to finish.
We weren't able to reach a character limit when entering some very lengthy captions, so feel free to make them as long as you want.