The present disclosure is directed to a method for monitoring focus of a photolithography system. The method comprises providing a substrate and depositing a photoresist layer on the substrate. At least one photomask is provided comprising one or more forbidden pitch photomask patterns formed thereon. The forbidden pitch patterns are imaged in the photoresist layer by exposing the photoresist layer to radiation through the at least one photomask. The imaged forbidden pitch patterns are developed in the photoresist. Focus error information regarding the imaging process can be determined using the developed forbidden pitch patterns.