University in analytic corrections to scaling for planar Ising models

It is argued that the leading corrections to scaling for planar Ising models, which occur as analytic factors, arise from the quadratic terms of the nonlinear thermal and ordering fields (rather than from irrelevant variables). This yields eff+2eff+eff=2 for the effective critical exponents, with no leading corrections, and is confirmed by exact square-lattice results for arbitrary anisotropy J2J1. For isotropic lattices the ratios of correction amplitudes and quadratic nonlinear-field terms appear universal.