  We report an effective functional form for the spin-spin correlation function
of the 2D Ising model as a function of temperature and field. Although the
Ising model has been well studied, no analytical result for the spin-spin
correlation function exists for arbitrary magnetic fields and temperatures. We
show the validity of our form by comparison with simulations using the Wolff
algorithm, and obtain remarkable precision by including analytic corrections to
scaling. Given recent interest in comparing biomembrane heterogeneity to Ising
criticality, our spin-spin correlation function may be used as a predictive
quantitative measure for FRET or NMR membrane experiments.
