Providing an optical system having excellent optical performance, an imaging apparatus, and a method for forming an image by the optical system. The optical system includes, in order from an object, a first lens group G1 having negative refractive power, a second lens group G2 having positive refractive power, a third lens group G3 having negative refractive power, and a fourth lens group G4 having positive refractive power. At least one of the first lens group G1, the second lens group G2, the third lens group G3, and the fourth lens group G4 has at least one A lens satisfying given conditional expressions.