We show that optical systems, from imagers to sophisticated metasurfaces, require a minimum thickness. Hence, for example, one thin metasurface layer may not be enough for many optical functions. This thickness arises whenever the input regions for different output pixels overlap. Even before starting design, just from what the optics is to do, we can deduce an overlapping non-locality number C that says how many channels must flow sideways inside the system. The system needs thickness to allow those channels to propagate sideways inside the structure. Several optimized system designs approach this limit but do not surpass it.
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