All opsins require retinal, a vitamin A‐related organic cofactor that serves as the antenna for photons, to function (Fig. 2A); when retinal is bound, the functional opsin proteins are termed rhodopsins. After retinal has diffused into the binding pocket of the seven-transmembrane-helix oval, it is covalently attached to a conserved lysine residue of helix 7 by formation of a protonated retinal Schiff base (RSBH+) (Fig. 2A). The ionic environment of the RSBH+, defined by the residues of the binding pocket, dictates the spectral and kinetic characteristics of each individual opsin. Upon absorption of a photon, retinal isomerizes and triggers a wave of conformational changes within the opsin partner. The photoisomerized retinal is the trigger for all subsequent structural rearrangements and all activities these proteins perform.