Oppo has a great idea for stuffing more cameras into your smartphone. Instead of having a camera lens aimed directly outwards, it's bending light by 90 degrees inside the chassis of the phone. Y'know, like a submarine's periscope.<
The stacked lens arrangement is actually oriented from side of the smartphone camera setup to the other -- light first enters the camera's lens through one forward-facing element (next to where it also hits a regular sensor through a wide-angle lens) and is then turned 90 degrees by an optical prism. This is a similar arrangement to a periscope, which uses either an optical prism or a flat mirror to reflect light at an angle.
Where a 2x zoom lens (it's not actually zoom since no elements move, but you get the point) is around 6mm thick, Oppo's 3x periscope-style setup is 5.7mm -- meaning phones with dual lens setups can be even slimmer than they currently are. It calls its new setup a 5x camera, though, because it also has "proprietary image fusion technology for digital zoom". We don't like digital zoom, but hey, Oppo, you do you.
The 3x telephoto lens, too, has optical image stabilisation applied to its final stacked element, massively improving shake reduction (by 40 per cent, apparently). versus sensor-based OIS methods. Oppo's engineers have filed 50-odd patents for the tech, and it's the result of a year's development work. Now we just have to wait for it to appear in a new handset. [Oppo]