  We show that a classical imaging criterion based on angular dependence of
small-angle phase can be applied to any system composed of planar, uniform
media to determine if it is a flat lens capable of forming a real paraxial
image and to estimate the image location. The real paraxial image location
obtained by this method shows agreement with past demonstrations of far-field
flat-lens imaging and can even predict the location of super-resolved images in
the near-field. The generality of this criterion leads to several new
predictions: flat lenses for transverse-electric polarization using dielectric
layers, a broadband flat lens working across the ultraviolet-visible spectrum,
and a flat lens configuration with an image plane located up to several
wavelengths from the exit surface. These predictions are supported by full-wave
simulations. Our work shows that small-angle phase can be used as a generic
metric to categorize and design flat lenses.
