Focus control plays important impact on the exposure quality in optical lithography. In order to ensure yield, exposure field needs to be in depth of focus all the time in the exposure process. The relationship between the total defocus of lithography system and yield is discussed by establishing a mathematical model, while the total defocus of lithography is the result of many defocus errors. This paper focuses on the focus control process of advanced dual-stage lithography system and digital micro-mirror device lithography system used for mask manufacturing, then analyzes their main defocus errors. Among the error factors, the focusing and leveling sensor has an important influence on the total defocus, since the focus control of lithography greatly depends on the accurate measurement of the focusing and leveling sensor. In this paper, the focusing and leveling sensors of Canon, Nikon, ASML, etc. are investigated. The working principles and structures of focusing and leveling sensors are compared.