Phytochromes are an important class of red/far-red responsive photoreceptors that act as light-activated biological switches, ultimately driving growth and development in plants, bacteria, and fungi. The composition of the red-absorbing ground-state has been widely debated due to the presence of a shoulder feature on the blue edge of electronic absorption spectra, which many have attributed to the presence of multiple ground-state conformers. Here we use resonance Raman intensity analysis to calculate the vibronic absorption profile of cyanobacterial phytochrome Cph1 and show that this shoulder feature is due simply to vibronic transitions from a single species, thus reflecting a homogeneous ground-state population.