  We report on the MILC Collaboration calculation of electromagnetic effects on
light pseudoscalar mesons. The simulations employ asqtad staggered dynamical
quarks in QCD plus quenched photons, with lattice spacings varying from 0.12 to
0.06 fm. Finite volume corrections for the MILC realization of lattice
electrodynamics have been calculated in chiral perturbation theory and applied
to the lattice data. These corrections differ from those calculated by Hayakawa
and Uno because our treatment of zero modes differs from theirs. Updated
results for the corrections to "Dashen's theorem" are presented.
