There are also single-pixel compressive sensing cameras, which don't need focusing optics at all, so they can be made lensless (and potentially flat). They also

There are also single-pixel compressive sensing cameras, which don't need focusing optics at all, so they can be made lensless (and potentially flat). They also have plenty of downsides though, such as poor performance in scenes with movement, or very high computational requirements for image reconstruction.