1. A focus masking structure used to determine the focus quality of a photolithographic pattern or a photolithographic system, said focus masking structure being disposed on a substrate, said focus masking structure comprising: a plurality of parallel source lines; and a plurality of phase shift zones configured to separate said plurality of parallel source lines, said plurality of phase shift zones alternating between a first phase shift zone and a second phase shift zone, said first and second phase shift zones having phases that are not equal to the phase of the substrate, and a phase difference therebetween that is not equal to 0 or 180 degrees or integer multiples of 180 degrees, wherein said plurality of parallel source lines and said phase shift zones cooperate to produce a focus pattern that includes at least two periodic structures that shift relative to one another with changes in focus, said shift being based on the sign and magnitude of defocus and the sign and magnitude of the phase difference.