Extracting (pion) form factors from finite volume lattices

In lattice QCD simulations, there is a strong tendency to over-estimate the form factors (and therefore, under-estimate the related charge radii) of pions and other hadrons.The three major systematic in lattice QCD: finite lattice spacing, heavier pion mass, and finite volume all could contribute to the over-estimation of the form factors.Using the chiral effective theory, this can be easily understood as a distortion of the chiral logarithm.If a chiral symmetric Dirac operator is employed for lattice QCD, one has a good control of the former two, while the volume has to be small due to its high numerical cost.In this talk, we focus on the finite volume effects using chiral perturbation theory and discuss how to avoid the over-estimation of the (pion) form factors.We also present our recent lattice results with dynamical chiral fermions.