  We draw an analogy between the chiral extrapolation of lattice QCD
calculations from large to small quark masses and the interpolation between the
large mass (weak field) and small mass (strong field) limits of the
Euler--Heisenberg QED effective action. In the latter case, where the exact
answer is known, a simple extrapolation of a form analogous to those proposed
for the QCD applications is shown to be surprisingly accurate over the entire
parameter range.
