We study the focusing of a Gaussian laser beam in a microscopic planar cavity when the laser wavelength is resonant in the cavity but the beam divergence is larger than the acceptance angle of the cavity. Using the luminescence of implanted praseodymium ions as a microscopic probe for the total electric field inside the spacer of the microresonator, we investigate theoretically and experimentally how strong focusing alters the photoexcitation of luminescent species located inside such a structure.