  In the present paper, we consider nonresonant corrections to $ 2s-ns/nd $
transition frequencies in hydrogen for the experiments based on two-photon
spectroscopy. A detailed study of angular correlations of quantum interference
effects within the framework of rigorous quantum electrodynamics is given.
Closed expressions for the resonant two-photon scattering cross sections on an
atomic level with dependence on all atomic quantum numbers including fine and
hyperfine structure are derived. These expressions are applied for the
description of experiments based on two-photon spectroscopy with fixing of
incident (outgoing) photon polarizations. We demonstrate that for certain
experimental geometry nonresonant corrections could be significant for the
determination of Rydberg constant and proton charge radius.
