This is a pet hate of mine. Since the introduction of HDR mode way back when, while using the Apple Camera app you can't keep HDR mode set to 'ON' permanently if you wish to. It goes off when you shut the camera off or go back to the lock screen. This is very annoying as you manually have to switch it on if you want to use it. Do you think Apple might implement this at some point in the future - seems so easy right?