A conversion lens which includes a pair of transparent members is provided. At least one of the transparent members is formed of a deformable film; a connecting member for connecting the pair of transparent members so as to form a sealed space sandwiched between the pair of transparent members; a liquid filled in the sealed space; and a curvature changing mechanism for changing the curvature of the deformable film by moving the liquid filled in the sealed space. A conversion lens system and an imaging device, provided with the conversion lens are also provided.