Laser radar is increasingly used to generate high resolution spatial maps of targets. The sensitivity, and hence utility, of laser radar (and laser rangefinders) depend on the laser cross section (LCS) of the target. A significant contribution to overall LCS can be the optical cross sections (OCSs) of its optical components. Optical systems, therefore, should be analyzed to quantify their OCSs. This work develops a better understanding of OCS measurements. Specifically, the measurements of a single-lens optical system with a substrate of known reflectivity at and near focus are discussed. Current practice fails to accurately predict the OCS of this setup; diffraction-limited theory is typically used near focus, where it predicts an OCS much greater than the measured value, and ray tracing is used as the substrate is moved away from focus, where it successfully captures the general trend but fails to predict deviations caused by diffraction and interference effects. We use a wave-optics approach to account for the effects of aberrations. A model is created to accurately predict the OCS of the single-lens system. The results, along with predictions of the current theory, are compared to experimental data. In general, wave optics produces more accurate OCS predictions than ray tracing.