We comment on the description of electromagnetic reactions involving hadrons, when the internal structure of the hadrons is taken into account. General off-shell vertex operators, only constrained by Lorentz and gauge invariance, are used. The electromagnetic production of pions on a nucleon is discussed as an example. Commonly used ad hoc recipes involving phenomenological form factors are discussed in the framework of an exact formulation.

• Received 2 November 1988

DOI:https://doi.org/10.1103/PhysRevC.39.1907