Open your camera instantly from any screen, even your lock screen, with Quick capture.
Twist your phone twice like this:

The 
Twist twice again to switch to the front (selfie) camera.
If you don’t want others to access your camera, you can turn Quick capture off. But if you use a screen lock, don’t worry--only your Camera app is accessible. To view existing photos or access any other features or apps, you must unlock your phone.
To turn Quick capture on or off: