In the case of a simple, one-piece optical imaging element the possibility of aplanatic correction is important. Such a correction can be obtained for a single holo-lens recorded on spherical surface. This needs, however, relatively high spatial frequency to be recorded. From the technological point of view the combination of classic spherical lens with a diffractive structure of relatively low spatial frequency seems much more convenient. Such hybrid lens is a single element consisting of diffractive and refractive parts. Even if one of the lens surfaces is flat (and on this surface the diffractive structure is deposited) it is possible to obtain the aplanatic correction for selected locations of the object point.
"Aplanatic correction of a hybrid lens", Proc. SPIE 2537, Novel Optical Systems Design and Optimization, (11 August 1995); doi: 10.1117/12.216396; https://doi.org/10.1117/12.216396