Optical imaging allows mapping the complex spatiotemporal dynamics of transmembrane potential and intracellular calcium in cardiac tissue. Several studies have shown that the epi-fluorescent optical action potential contains contributions from the sub-epicardium, owing to scattering of photons in tissue. Hybrid electro-optical models have allowed careful quantification of these scattering effects and have lead to a better interpretation of the optical action potential. However, until now, these effects have not been investigated for optically recorded calcium transients. Here, we develop a hybrid model of cardiac dual calcium-voltage epi-fluorescence mapping. This model allows simulating both optical action potentials and optical calcium transients and investigating the effects of photon scattering on their synthesis. We find that optical calcium transients contain contributions from sub-epicardial layers up to 0.8 mm below the epicardium. These lead to significant differences in rise time and activation times between the optically acquired calcium signal and the epicardial intracellular calcium concentration. As has been the case with optically recording action potentials, these results should be taken into account in the interpretation of experimental optical measurements of intracellular calcium.