With the LG G3, holding the volume down key opens up the camera app. It's reasonably quick, but it took me some time to get used to the button placement as they're on the back of the phone. The volume down key is also right next to the power on/off button.
It's a similar story when you actually want to take the photo: a short push of the down key will take the shot, but if you miss, you end up switching the lock key instead, which quickly gets infuriating.
It's also difficult to hold the phone as a normal camera with this kind of set up, your hand having to obscure the screen, or hold it very awkwardly, if you want to use the physical button. It's probably best to stick with the onscreen virtual buttons.
LG seems to have gone for an iPhone style very minimal amount of control type setup with the G3, not allowing you to change any of the camera's key settings within the native app.
The G3 uses laser autofocusing, which makes it one of the quickest on test here, and it impressively locks onto a target, and in low light it still works well. That said, for macro focusing, it's much more disappointing. If you're shooting something small, or with very fine detail, it struggles to lock on at all.
When you can get the G3 to focus, the images it produces are great with lots of detail and punchy colours… in good light. In low light, the images produced aren't great, but certainly good enough for sharing online, there's a little smudging to be seen when viewed at normal sizes.
The digital zoom is the most disappointing of the group here, with a big loss of detail at the zoom end of the range, so it's best to avoid using this unless really necessary. On the plus side, the front facing camera is one of the better ones of the group, working reasonably well in low lighting conditions.