  We initiate a systematic method to calculate both the finite volume energy
levels and form factors from the momentum space finite volume two-point
function. By expanding the two point function in the volume we extracted the
leading exponential volume correction both to the energy of a moving particle
state and to the simplest non-diagonal form factor. The form factor corrections
are given in terms of a regularized infinite volume 3-particle form factor and
terms related to the L\"usher correction of the momentum quantization. We
tested these results against second order Lagrangian and Hamiltonian
perturbation theory in the sinh-Gordon theory and we obtained perfect
agreement.
