  We consider the influence of photon-initiated processes on high-mass particle
production. We discuss in detail the photon PDF at relatively high parton $x$,
relevant to such processes, and evaluate its uncertainties. In particular we
show that, as the dominant contribution to the input photon distribution is due
to coherent photon emission, at phenomenologically relevant scales the photon
PDF is already well determined in this region, with the corresponding
uncertainties under good control. We then demonstrate the implications of this
result for the example processes of high-mass lepton and $W$ boson pair
production at the LHC and FCC. While for the former process the
photon-initiated contribution is expected to be small, in the latter case we
find that it is potentially significant, in particular at larger masses.
