The oscillation of a cantilever in ultrahigh vacuum dynamic force microscopy is sometimes measured with the help of a Fabry-Perot interferometer. We show that the photoinduced forces present in such an interferometer can artificially increase or decrease the effective quality factor of the cantilever. We examine this effect on the basis of a PtIr-coated rectangular silicon cantilever at temperatures between 7.7 and 30 K. By measuring resonance curves we demonstrate that the magnitude of the effective quality factor changes with cantilever-fiber distance, laser power, and temperature.