Metalenses are perfect lenses, at least they could be. In reality all kinds of fabrication imperfections and assembly errors reduce their performance. For real-life applications robust designs are needed so that the fabricated lenses perform within tolerance. Producers have to know which imperfections are critical and which deviations are acceptable.
This interesting paper discusses the effect of changes in a nano-pillars on their performance in a cylindrical lens. The authors examine how the intercept factor changes with random imperfections in the radius and flatness of the nano-pillars.
It made me wonder what the effect of all other kinds of imperfections are such as steepness of the sidewalls, warping of the substrate or roughness of the material.
Optical modelling tools are essential for tolerancing and robustness studies.