A method for scanning a microarray, including (a) providing a microarray to an optical scanner, wherein the microarray includes a surface having features having different target molecules; (b) scanning the surface in the x or y direction to acquire optical signals from the features at sequential regions of the surface, wherein the focus setting between the optical scanner and the surface is dynamically controlled in real time by repeatedly (i) adding a predetermined focal offset, thereby introducing an error in the focus setting, and (ii) adjusting the focus setting to reduce the error in the focus setting, thereby acquiring the optical signals from different regions at different degrees of focus such that a subset of the regions that are scanned have an introduced focus error; and (c) analyzing the optical signals that are obtained at the different degrees of focus to identify the different target molecules.