  We consider a microscopy setting where quantum light is used for
illumination. Spontaneous parametric down conversion (SPDC) is used as a source
of a heralded single photon, which is quantum light prepared in a Fock state.
We present analytical formulas for the spatial mode tracking along with the
heralded and non-heralded mode widths. The obtained analytical results are
supported by numerical calculations and the following discussion taking into
account realistic setup parameters such as finite-size optics and finite-size
single-photon detectors. This allows us to observe that the diffraction limit
can be approached with simultaneous alleviation of the photon loss leading to
increased signal-to-noise ratio - a factor limiting practical applications of
quantum light. Additionally, it is shown that the spatial resolution can be
manipulated by carefully preparing the amplitude and phase of the spatial mode
profile of the single photon at the input to the microscope objective. Here,
the spatial entanglement of the biphoton wavefunction or adaptive optics can be
applied for spatial mode shaping. Analytical dependencies between the incident
and focused spatial mode profiles parameters are provided.
