1. A measurement method, comprising: a first process of exposing an object by generating a first pattern image on the object via an optical system; a second process of exposing a part of an area on the object where the first pattern image has been generated via the optical system; a third process of measuring a pattern image that is formed on the object after processing of the second process using a measurement device; and a fourth process of computing an optical characteristic of the optical system using a measurement result of the pattern image.