CeF_3 and PrF_3 single crystals are investigated as Faraday rotators for the UV-visible region. Their properties are compared with those of the industrial standard reference terbium-gallium-garnet (TGG) single crystal. In contrast to TGG, CeF_3 exhibits a higher transparency in the whole near UV-visible-IR, and a remarkable higher figure of merit which rapidly increases towards the cutoff. In the case of PrF_3, the transparency extends to even shorter wavelengths, and analogously to CeF_3 its figure of merit increases notably in the UV. These results indicate the potential of CeF_3 and PrF_3 as optical isolators in the UV-visible, where at present there are no alternative candidates.