This study investigates the size-dependent optical properties of colloidal PbS quantum dots (Qdots) by combining their absorbance spectra with detailed elemental analysis of the Qdot suspensions. The molar extinction coefficient (ε) increases with the Qdot volume (d³) and agrees with theoretical calculations using the Maxwell-Garnett effective medium theory and bulk values for the Qdot dielectric function. This demonstrates that quantum confinement has no influence on ε in this spectral range, and it provides an accurate method to calculate the Qdot concentration. Around the band gap, ε only increases with d¹·³, and values are comparable to the ε of PbSe Qdots. The data are related to the oscillator strength (f_if) of the band gap transition and results agree well with theoretical tight-binding calculations, predicting a linear dependence of f_if on d. For both PbS and PbSe Qdots, the exciton lifetime (τ) is calculated from f_if. Values range between 1 and 3 μs, in agreement with experimental literature data from time-resolved luminescence spectroscopy. The results provide a thorough general framework to calculate and understand the optical properties of suspended colloidal quantum dots. Most importantly, it highlights the significance of the local field factor in these systems. The study also shows that the optical properties of PbS Qdots at energies well above the band gap are not influenced by quantum confinement, which has important practical implications for determining Qdot concentration. The results confirm the applicability of the Maxwell-Garnett effective medium theory and provide insights into the underlying physics determining the optical properties of colloidal quantum dots.This study investigates the size-dependent optical properties of colloidal PbS quantum dots (Qdots) by combining their absorbance spectra with detailed elemental analysis of the Qdot suspensions. The molar extinction coefficient (ε) increases with the Qdot volume (d³) and agrees with theoretical calculations using the Maxwell-Garnett effective medium theory and bulk values for the Qdot dielectric function. This demonstrates that quantum confinement has no influence on ε in this spectral range, and it provides an accurate method to calculate the Qdot concentration. Around the band gap, ε only increases with d¹·³, and values are comparable to the ε of PbSe Qdots. The data are related to the oscillator strength (f_if) of the band gap transition and results agree well with theoretical tight-binding calculations, predicting a linear dependence of f_if on d. For both PbS and PbSe Qdots, the exciton lifetime (τ) is calculated from f_if. Values range between 1 and 3 μs, in agreement with experimental literature data from time-resolved luminescence spectroscopy. The results provide a thorough general framework to calculate and understand the optical properties of suspended colloidal quantum dots. Most importantly, it highlights the significance of the local field factor in these systems. The study also shows that the optical properties of PbS Qdots at energies well above the band gap are not influenced by quantum confinement, which has important practical implications for determining Qdot concentration. The results confirm the applicability of the Maxwell-Garnett effective medium theory and provide insights into the underlying physics determining the optical properties of colloidal quantum dots.