where σ σ μ μ ν ν = ( i / 2 ) [ γ γ μ μ , γ γ ν ν ] , q ν ν is the incoming four-momentum of the external photon (on the right-hand side of the figure), and F 1 (q) and F 2 (q) are form factors that depend only on the momentum transfer q. At tree level (or leading order), F 1 (q) = 1 and F 2 (q) = 0. Beyond leading order, the corrections to F 1 (0) are exactly canceled by the field strength renormalization. The form factor F 2 (0) corresponds to the anomalous magnetic moment a of the fermion, defined in terms of the Landé g-factor as: