  Experimental conclusions from air shower observations on cosmic-ray photons
above 10^19 eV are based on the comparison to detailed shower simulations. For
the calculations, the photonuclear cross-section needs to be extrapolated over
several orders of magnitude in energy. The uncertainty from the cross-section
extrapolation translates into an uncertainty of the predicted shower features
for primary photons and, thus, into uncertainties for a possible data
interpretation. After briefly reviewing the current status of ultra-high energy
photon studies, the impact of the uncertainty of the photonuclear cross-section
for shower calculations is investigated. Estimates for the uncertainties in the
main shower observables are provided. Photon discrimination is shown to be
possible even for rapidly rising cross-sections. When photon-initiated showers
are identified, it is argued that the sensitivity of photon shower observables
to the photonuclear cross-section can in turn be exploited to constrain the
cross-section at energies not accessible at colliders.
