Measurement method, inspection apparatus, patterning device, lithographic system and device manufacturing method

A method comprising:

• using a lithographic apparatus to print at least a first focus metrology target on a substrate, the printed focus metrology target including at least a first periodic array of features;
• using inspection radiation to obtain one or more diffraction signals from the first periodic array in the printed focus metrology target;
• and
• deriving a measurement of focus performance of the lithographic apparatus based at least in part on the diffraction signals,
• wherein the first periodic array comprises a repeating arrangement of first zones interleaved with second zones in at least a first direction of periodicity, a feature density being different in the first zones and the second zones, and
• wherein each of the first zones includes a repeating arrangement of first features, a minimum dimension of each first feature being close to but not less than a resolution limit of the printing step.