This study presents wavelength-dependent mass absorption cross sections (MAC) and calibration factors for filter-based photometers derived from controlled laboratory experiments with well-characterized aerosols. Using soot particles with varying coating thickness, MAC is evaluated as a function of single scattering albedo (SSA). Compared to common core-shell model predictions, MAC can grow less or even decrease with increasing coating, suggesting that morphological changes, such as restructuring and non-ideal mixing states, can suppress the expected lensing effect. Comparisons with optical models indicatebetter agreement with fractal and porous particle representations than with idealized core–shell structures. These findings emphasize the importance of particle morphology and growth pathways for interpreting aerosol absorption and improving photometer calibration. The wavelength dependence of the MAC agrees better with expectations, since the higher imaginary part of the refractive index in the UV, compared to the red or near-infrared wavelengths of organic matter, leads to stronger UV absorption, independent of morphological effects.