Focus generally means the focal point of a lens/lens group, that is the point with the sharpest image. In relation to an image recording system, this is the optimal focus setting of a lens, respectively lens system. From photography we know that the focus interval before and after the optimal focal point may be bigger or smaller, depending on the shutter opening of the lens (focus depth). For industrial image processing it is, once more, decisive, to be able to hold in focus a work span that is as large as possible when the purpose of optical recording is e.g. the reliable check-up of a welding seam by means of a laser head. Three-dimensionally recorded objects require thus also a higher focus depth than the ones that are processed two-dimensionally.
The setting process of the lens/lens system to the optimal distance from the object is called focussing. Focussing can be performed one time only through a calibration process for rigid systems, however, better known from photography are systems that can perform this task variably, either through manual control or through appropriate automatic processes (“auto-focus”).