  Chromatic aberration in optical systems arises from the wavelength dependence
of a glass's refractive index. Polychromatic rays incident upon an optical
surface are refracted at slightly different angles and in traversing an optical
system follow distinct paths creating images displaced according to color.
Although arising from dispersion, it manifests as a spatial distortion
correctable only with compound lenses with multiple glasses and accumulates in
complicated imaging systems. While chromatic aberration is measured with
interferometry, simple methods are attractive for their ease of use and low
cost. In this letter we retrieve the longitudinal chromatic focal shift of high
numerical aperture (NA) microscope objectives from the extinction spectra of
metallic nanoparticles within the focal plane. The method is accurate for high
NA objectives with apochromatic correction, and enables rapid assessment of the
chromatic aberration of any complete microscopy systems, since it is
straightforward to implement
