Femtosecond laser excitation of strongly absorbing thin films generates picosecond acoustic waves in the surrounding medium. In time-domain transient absorption experiments, these waves can give rise to Brillouin oscillations. In this contribution, the attenuation of the Brillouin oscillations in transient absorption microscopy experiments has been investigated for different excitation and detection conditions. The results show that the measured attenuation constants strongly depend on the numerical aperture (NA) of the microscope objective used in the experiments, with higher NA objectives giving larger attenuation constants. Modeling the effect using Gaussian beam theory shows that the increased attenuation at high NA arises from diffraction effects in both the light fields and the acoustic waves. These results are important for understanding the spectral resolution in imaging applications of the Brillouin scattering effect.