Why Does My iPhone 5s Back Camera Having Focus Problems? Here's The Fix.
When trying to focus on an object or person, it may help to tap the screen where the object is, this helps set the focus and exposure. You also may want to see if your camera has any smudges on it.
Is this a new issue or long term? Since your device is most likely not covered by insurance or warranty because it’s a few years old, any repair would be out of pocket. It sounds like you may need your camera replaced.