The new cinematic mode "anticipates when someone is about to enter the frame" and shifts focus to them, Apple said - something known as "pull focus".
It is the only smartphone that lets users edit this effect after shooting, Apple boss Tim Cook said.
However, most other features on the new model were incremental updates over previous versions.
The event was also overshadowed by news of a new security flaw in Apple devices which could expose users' messages.
Apple released a security patch on Monday for the previously-unknown flaw, which could let attackers access its iMessage service without the user clicking on a malicious link or file.