Laboratory calibration of a beam transmissometer by using natural sedimentary particles fractionated into seven size classes demonstrates that slopes of light attenuation vs. particle mass concentration regressions systematically vary with the size distributions of the particles. Suspensions with a mean particle size of 8.5 μm attenuate 660-nm light 15 times more efficiently than suspensions of similar particles with a mean diameter of 48 μm. Theoretical calculations of attenuation, based on scattering by spherical particles with the measured size distributions, underestimate observed attenuation by factors of 2--4, apparently because the nonspherical shape and roughness typical of natural particles create an effective optical diameter larger than that for a sphere of equal volume. These laboratory results agree with the trend established from a survey of published field data showing that calibration slopes experience a steady increase from inshore high-energy waters, where large particles are common, to offshore low-energy waters, where large particles are rare.