Creating a single photon to a light field or removing a single photon from a light field reveals interesting quantum phenomena. For theoretical modeling and understanding of these experiments it is essential to be able to model the coupling of the optical field to the measurement apparatus. We investigate a cascaded field–quantum system–reservoir setup to obtain a theoretical model that can, in contrast to previous models, simultaneously model both the weak and the strong coupling regimes of the field–detector system. Furthermore, our theory can be applied to model optical fields coupled to dissipative systems or to active amplifying systems including for example operation of various optical instruments and devices, such as detectors, LEDs and lasers.