Retrieval of the aerosol size distribution from optical measurements at ground level is well known to be a difficult problem. Nowadays objective techniques that can give a solution without the intervention of the researcher do not exist. We propose several objective methods that are well based in the mathematical and physical points of view. Their accuracy is evaluated and the top performance of the objective inversion techniques is presented. Moreover physical and experimental suggestions can be drawn to improve the accuracy. Inversions with experimental optical depths are also shown.