G protein-coupled photoreceptor that signals through the G(q/11) family of G proteins in response to light in intrinsically photosensitive retinal ganglion cells (ipRGCs) (By similarity). Can also activate other G proteins, including G(i/o) and G(s), depending on the cell type, leading to either activation or inhibition of adenylate cyclase and cAMP-dependent signaling (PubMed:35274137). Activation occurs when opsin-bound cis-retinal chromophore absorbs a photon and isomerizes to all-trans-retinal, inducing a conformational change in the opsin that triggers a G protein-mediated phototransduction cascade (PubMed:15674244). Acts as a bistable protein, allowing photoreversible regeneration of the cis-retinal chromophore after stimulation (PubMed:15674244). Contributes to non-image-forming visual response to light in ipRGCs, including the pupillary light reflex and the regulation of circadian rhythms (By similarity). In M1 ipRGCs (non- image forming) subtypes, activates a G(q)-phospholipase C (PLC) signaling cascade leading to depolarization through opening of TRPC6/7 channels and decreased intrisic excitability (By similarity). Mediates phototransduction in M4 ipRGCs (image forming) subtypes across dim to bright light conditions via G(q)-PLC signaling, resulting in increased intrisic excitability through closure of potassium leak channels (most likely TASK channels), thereby enhancing contrast sensitivity (By similarity). At bright light intensities, further enhances depolarization in M4 ipRGCs via activation of TRPC channels (By similarity). May be involved in the optokinetic visual tracking response. May also play a role in regulating retinal hyaloid vessel growth and regression (By similarity). {ECO:0000250|UniProtKB:Q9QXZ9, ECO:0000269|PubMed:15674244, ECO:0000269|PubMed:35274137}. [Isoform 2]: In contrast with isoform 1, cannot activate G(i/o) and G(s) family of G proteins in response to light. . This is the function of OPN4 (opsin 4, Ensembl gene identifier ENSG00000122375).