https://techcrunch.com/2020/09/18/mit-engineers-develop-a-totally-flat-fisheye-lens-that-could-make-wide-angle-cameras-easier-to-produce/

Engineers at MIT, in partnership with the University of Massachusetts at Lowell, have devised a way to build a camera lens that avoids the typical spherical curve of ultra-wide-angle glass, while still providing true optical fisheye distortion. The fisheye lens is relatively specialist, producing images that can cover as wide an area as 180 degrees or more, but they can be very costly to produce, and are typically heavy, large lenses that aren’t ideal for use on small cameras like those found on smartphones.

This is the first time that a flat lens has been able to product clear, 180-degree images that cover a true panoramic spread. The engineers were able to make it work by patterning a thin wafer of glass on one side with microscopic, three-dimensional structures that are positioned very precisely in order to scatter any inbound light in precisely the same way that a curved piece of glass would.