The paper brings results of far infrared transmission studies of relaxor ferroelectric PbMg1/3Nb2/3O3 and PbSc1/2Ta1/2O3 thin films performed at temperatures between 10K and 900K. The spectra clearly show that the lowest-frequency polar optic phonon softens (partially) neither towards the temperature of dielectric-constant maxima, nor towards the freezing temperature, but towards the Burns temperature. This indicates that it represents the ferroelectric soft mode inside the polar clusters. The dielectric relaxation, which expresses the dynamics (flipping and breathing) of polar clusters, appears in THz range below the Burns temperature. This relaxation dramatically slows down to sub-Hertz region on cooling to freezing temperature near 200 K (in PMN) or to T-c (in PST). Another heavily damped excitation remains in the spectra below 30 cm(-1) down to 10K. This mode was assigned to the E component of the split Flu soft mode.