Image formation is the process where radiation emitted from objects is collected to form an image of the objects in one or the other way. Two principle types of image formation can be distinguished. With direct imaging, rays emitted from one point of an object are — except for possible aberrations — collected in one point at the image plane. This is not the case with indirect imaging. Here, a point at the image plane may collect a radiation from various points of the image objects. An image of the object can only be formed after a more or less complex reconstruction process.