The iPhone camera app has excellent point and shoot functions, however if you want to gain more control over your photography, you will want to have more control than that, and you can. There is an exposure and lock feature which allows you to lock the parameters and frame your shot to look great.
In order to activate the AE/AF lock so you can lock exposure and focus the iPhone camera app all you have to do is open the app and tap on the screen as if you were going to focus on an area. However, instead of just tapping and releasing, tap and hold until the focus square flickers. When you release, you should see an “AE/AF Lock” indicator on the screen.
When the AE/AF Lock is enabled, you can frame your photo with no risk of losing the exposure and focus controls you set. When you want to undo the exposure and focus lock, just tap on the screen once and it will be removed.