Light-exposed organisms developed photoreceptors to transduce light signals for environmental adaptation. Phytochromes, found in bacteria, fungi, and plants, can discriminate the ratio of red and far-red light using the isomerization of a bilin chromophore bound to a photosensory module to trigger down- stream conformational changes in the protein. Here, we investigated by hydrogen/deuterium exchange mass spectrometry and electron paramagnetic resonance spectroscopy the light-driven domain mechanics of a minimal monomeric photosensory module from the group II phytochrome Cph2 from Synechocystis sp. PCC 6803. We could unambigu- ously trace the light-driven secondary structural rearrangement of its tongue region, and we found a translational motion of the PHY domain that is related to what was found before by X-ray studies in a group I module. Our analysis demonstrates a common light response in the photosensory modules of phyto- chromes, orchestrated solely by the GAF-PHY bido-main independent of further quaternary interactions or the nature of downstream effector domains.