The fluorescence collection from single atoms and emitters has been extensively utilized in quantum information and quantum optics research. Here, we investigated the collection efficiency of an objective lens by drawing an analogy between the free-space beam (FSB) and a waveguide mode. We explored how efficiency is influenced by their thermal motion within a dipole trap. Furthermore, we introduce an effective energy fraction ratio to quantify potential imperfections in the focusing of the objective lens. Our results provide valuable insights for optimizing the fluorescence collection in single-atom experiments and highlight the importance of considering realistic experimental conditions when estimating achievable efficiencies.