A method and apparatus for obtaining focus information relating to a lithographic process. The method includes illuminating a target, the target having alternating first and second structures, wherein the form of the second structures is focus dependent, while the form of the first structures does not have the same focus dependence as that of the second structures, and detecting radiation redirected by the target to obtain for that target an asymmetry measurement representing an overall asymmetry of the target, wherein the asymmetry measurement is indicative of focus of the beam forming the target. An associated mask for forming such a target, and a substrate having such a target.