We analyze the impact of the population lens on electromagnetic (EM) field development in a Q -switched unstable supergaussian cavity (QUSC) by a FFT based computational model. The results show distortions in a structure of the near field as well as of the far-field pattern and reveal detrimental impact on the capability of energy extraction from the gain material and the energy stability of pulses. The computational results were confirmed with the experimental analysis of a ruby laser with QUSC. The computational results are also in excellent agreement with the published work dealing with the Cr:LISAF QUSC cavity. Our investigations show that population lens should be taken into account when using chromium-doped laser media in a QUSC.