A system includes an illuminator, a mask, and a measurement device. The illuminator includes a light source. The mask includes at least one focus determination pattern having a first pattern portion and an adjacent second pattern portion. The first pattern portion and the second pattern portion have substantially the same width but produce a phase difference in light transmitted through the pattern portions. The measurement device measures a first critical dimension and a second critical dimension of a feature produced on a target by the at least one focus determination pattern. The difference between the first critical dimension and the second critical dimension relates to an amount of defocus and is sensitive to the focus change. The system may also include a feedback control loop where a determination regarding an amount of defocus is used to focus the position of a wafer or a mask or both of them onto the target. Additional apparatus, systems, and methods are disclosed.