Calculated bandwidth bounds for wide-FOV dispersion-engineered achromatic metalenses. In (a) and (c), the bound is calculated for a focal spot location given by r₀ = Ftan(α) and in (b) and (d) for r₀ = Fα. To represent the bound in a general way applicable to vastly different metalenses, the bandwidth is normalized by the factor κ, the focal length of the metalens F, and the speed of light c. The bandwidth axis in (a) and (b) has been scaled logarithmically for clarity. In all the panels, NA, n_b, and FOV are the numerical aperture, background refractive index, and the field of view, respectively.