In this paper, the optical depths in blazars due to photo-pair production is calculated for a time-dependent, non-linear injection model. Several target photon fields are taken into account, namely the internal synchrotron, synchrotron self-Compton and external Compton radiation, as well as a constant external soft photon field. By applying the optical depths to theoretical blazar spectra, only the constant external photon field turns out to significantly influence the radiation at high energies. The impact of the internal time-dependent radiation fields is either minor or requires extreme parameter settings. Additionally, the synchrotron self-absorption turn-over energy for low synchrotron energies is calculated, which is inherently time dependent. It would be challenging to use it to constrain free parameters, since precise knowledge of the observation time relative to the injection time is needed. In conclusion, optical depth does not significantly influence the non-linear, time-dependent injection and cooling model.