At least one exemplary embodiment is directed to an optical system which includes a compound optical element including a lens element and a resin layer having different optical characteristics and having an aspherical interface therebetween. When Rref is a reference radius of curvature of the interface within an effective diameter, L is a distance from a diaphragm, ndg and .nu.dg are a refractive index and an Abbe number, respectively, of a material of the lens element at the d-line, and ndj and .nu.dj are a refractive index and an Abbe number, respectively, of a material of the resin layer at the d-line, the following expressions are satisfied: -1.5