Rod-specific cGMP phosphodiesterase that catalyzes the hydrolysis of 3',5'-cyclic GMP (By similarity). Necessary for the formation of a functional phosphodiesterase holoenzyme (PubMed:1847109). Involved in retinal circadian rhythm photoentrainment via modulation of UVA and orange light-induced phase-shift of the retina clock (PubMed:30240620). May participate in processes of transmission and amplification of the visual signal (By similarity). 3',5'-cyclic GMP + H2O = GMP + H(+) Binds 2 divalent metal cations per subunit. Site 1 may preferentially bind zinc ions, while site 2 has a preference for magnesium and/or manganese ions. Oligomer composed of two catalytic chains (alpha and beta), an inhibitory chain (gamma) and the delta chain. Defects in Pde6b are the cause of retinal degeneration. Abolishes the retinal photoreceptor layer, outer nuclear layer, and outer plexiform layer in the retinal ultrastructure (PubMed:30240620). Loss of Opn1sw, Opn1mw and Rho expression in the retina (PubMed:30240620). Reduced rate of circadian photoentrainment, UVA and orange light-induced phase-shift response and Fos expression in the suprachiasmatic nuclei (SCN) in the brain (PubMed:30240620). Pde6b and Opn5 double knockout mice also show loss of retinal ultrastructures and a more severe reduction in the rate of circadian photoentrainment, light-induced phase-shift response and Fos expression in the SCN (PubMed:30240620). Belongs to the cyclic nucleotide phosphodiesterase family.