  Two key metrics for imaging systems are their magnification and optical
bandwidth. While high-quality imaging systems today achieve bandwidths spanning
the whole visible spectrum and large changes in magnification via optical zoom,
these often entail lens assemblies with bulky elements unfit for
size-constrained applications. Metalenses present a methodology for
miniaturization but their strong chromatic aberrations and the lack of a
varifocal achromatic element limit their utility. While exemplary broadband
achromatic metalenses are realizable via dispersion engineering, in practice,
these designs are limited to small physical apertures as large area lenses
would require phase compensating scatterers with aspect ratios infeasible for
fabrication. Many applications, however, necessitate larger areas to collect
more photons for better signal-to-noise ratio and furthermore must also operate
with unpolarized light. In this paper, we simultaneously achieve achromatic
operation at visible wavelengths and varifocal control using a
polarization-insensitive, hybrid optical-digital system with area unconstrained
by dispersion-engineered scatterers. We derive phase equations for a pair of
conjugate metasurfaces that generate a focused accelerating beam for chromatic
focal shift control and a wide tunable focal length range of 4.8 mm (a
667-diopter change). Utilizing this conjugate pair, we realize a near
spectrally invariant point spread function across the visible regime. We then
combine the metasurfaces with a post-capture deconvolution algorithm to image
full-color patterns under incoherent white light, demonstrating an achromatic
5x zoom range. Simultaneously achromatic and varifocal metalenses could have
applications in various fields including augmented reality, implantable
microscopes, and machine vision sensors.
