Hi,When the screen is off you can just keep the camera button pressed for a short moment and it will activate the camera. You can choose from the original camera app(have a modified version of it to go with LineageOS as none of the camera apps that I tried didn't have any settings for it) if you want that button to just to launch the camera app, launch the app and take a picture or launch the app and start recording a video. I loved how fast and easy it is to snap a fast picture.
I've tried now Tasker and Button Mapper apps(used root for both) but neither worked from the screen off. With LineageOS(+Gapps Full) the button itself works by default when the screen is on and launches the camera app but quite randomly and not from the lockscreen.. Reason why it worked randomly sometimes I believe is because I didn't have the camera app running every time background; was quite distracted when I tried everything I could think of often at the same time. :silly: Not sure if the camera button for the pictures itself worked the same as originally though as it should first focus and if you keep it pressed then it takes the picture. With Tasker and Button Mapper sometimes it couldn't even take a picture as it seems to work as two different features. In Button Mapper if you bind the camera key it's called Camera. If you keep it pressed few seconds the key gets Shutter name. So it's first Camera button and turns in few seconds to a Shutter button? Seems so or perhaps it was just Button Mappers thing.
Anyhow I don't know much of these things so that's all I can explain probably.
did you try Settings->System->Gestures->Jump to camera? This will open camera if you fastpress the power button twice in standby.
Strang thing is, if i search in settings for camera i got some results, the first one is named 'Camera button' under Settings->System->Buttons, but if i click on that result it does not lead to the right setting...
There is also one result called 'Launch Camera' - ...A long press and release will launch camera...