Greetings, MarnixWalinga.
Welcome to the Apple Support Communities. Your camera is an important tool, so we can understand why you’d want to resolve the issue with .5 zoom not working as expected.
First, Restart your iPhone and force the Camera app to quit: How to force an app to close on your iPhone, iPad, or iPod touch
If the issue continues, work through these steps: If the camera or flash on your iPhone, iPad, or iPod touch isn't working
If the issue continues, test to see if it happens in third-party camera apps as well.
All the best.