Did you know there’s a shortcut on the Lock screen to quickly open the camera? Swipe from right to left across the Lock screen. Bear in mind you can’t return to the Lock screen by swiping back the other way – you’ll need to swipe on the Home bar or press the Home button instead.
You can also access the camera by long-pressing on the Camera icon in the bottom right of the Lock screen. Note that nothing happens if you quickly press it (to avoid it activating by mistake in your bag or pocket).
For security, if the Camera app is opened in this way, only the photos just taken can be seen; other items in the Camera Roll won’t be accessible.