Where is the camera button on my iPhone iOS 5 lock screen?

One of the features highlighted by Apple in iOS 5 is the addition of a camera button on the lock screen. This makes it possible to quickly access the Camera app without having to unlock the iPhone and find the icon on your home screen.

You may be wondering why this button isn't appearing on your lock screen. There are no camera or photo settings to turn on the camera unlock button. To make the button appear, you have to double-tap the home button while the iPhone is locked. The camera button will then appear.

Tapping the camera button will open the Camera app and allow photos and video to be taken, but if the iPhone is locked the camera roll can't be accessed.