I seem to be Having the same problem in the past few weeks. I take a photo then go back to open it and the phone starts shutting down on 35% - 40% power. In some cases the Phone Freezes with the camera mode with a blurry picture and I'm unable to shut it down or get rid of it so i just leave it on the bench for ages. Very frustrating coz i need it for work. One of my work colleagues went to a repair shop with the similar problem and a Technician said they're programmed to get faulty after about 2 - 2.5 years so that you upgrade to the newest version.
I've had the same problem too. When the phone shuts down, my only fix is to do the reset thing (hold both buttons down for ages). The screen will show that you need to plug it in to a charger but just keep holding the buttons down. Screen will go black again and then the Apple logo will show up, same as a normal start up. Might take a couple of goes but it does eventually work and you get your battery indicator showing the correct amount left.
Yep, i've been experiencing this for ages on my iphone 5. Im restoring my car in the garage, so taking a lot of photos of the progress, and as soon as the temperature dips to mid teens no matter what percentage the batterys on can gaurantee it will die. Warm the phone up a bit and can eventually get it to turn back on, and will display a healthy battery percentage until using the Apple camera app again. is it the app, battery or software? ,might try a third party camera app and see if the same thing happens!