Give yourself a lock screen shortcut, and use “sightseeing mode”

The HTC One -- and many other HTC Sense devices -- make it easy to jump straight into the camera application.  If you keep the camera app in the app dock at the bottom of the screen, you’ll be able to drag this up and immediately launch the camera app from the lock screen.

HTC Sense’s “sightseeing mode” also means you can easily jump in and out of the app at ease. Lock the screen at the camera app, then the second you press the power button again, you’ll bypass the lock screen and jump back into the camera app. As the name suggests, that’s ideal if you’re out and about shooting lots of pictures of different stuff.

Both of these tricks will bypass any lock screen security you have set up, but to do anything outside of the camera app you’ll still need to enter your pattern or PIN lock.