Provided are a composition containing particles having a refractive index of 2.0 or more and an average primary particle diameter of 200 nm or less, a film-forming component, and a solvent, in which the film-forming component includes two or more kinds of resins, or includes one or more kinds of resins and one or more kinds of polymerizable monomers, and at least one kind of the resins includes a resin a having a partial structure represented by Formula (1); a film formed of the composition; an optical sensor; and a resin including at least one repeating unit selected from a repeating unit represented by Formula (1-1) or a repeating unit represented by Formula (1-2).