Photodetachment of negative ions in combined laser and low-frequency fields is investigated. The time-dependent Green's function method is used for calculation of electron flux at a macroscopic distance from the photodetachment source, typical for a photodetachment microscopy experiment. In calculating the electron flux, we use the stationary phase method for the time integral, equivalent to the semiclassical approximation, to compute the time-dependent wave function. The stationary points ${t}_{1}^{(i)}, i=1,...,n$ correspond to time instances of launching of classical trajectories arriving at the detector at a given spacetime point $(\mathbf{r},t)$. The number of trajectories $n$ contributing to the electron flux at any point in the classically allowed spacetime domain can be controlled by varying the switching interval of the high-frequency laser which initiates the photodetachment process. The divergences inherent in the electron flux in the semiclassical treatment are removed by using the uniform Airy approximation near the caustics.