An inspection method is provided to determine a value related to a parameter of a target pattern printed on a substrate by a lithographic process used to manufacture a device layer on a substrate. The inspection method can include using an optical system with a high-NA objective lens, where the high-NA objective lens includes an object plane and a pupil plane. The inspection method can also include providing an aperture member to define at least one obscuration, determining a radial distance between a radially innermost point of each dark area and a nominal center of an image in a pupil plane, and determining an axial distance between the target and an object plane from the determined radial distance.