1. A method for focusing a beam of electrically-charged particles in a particle-optical device with an imaging objective lens, comprising the steps: (a). forming from the aforementioned beam (that is to say the beam of electrically charged particles that is to be focused in the particle-optical device), a first astigmatic beam with an attendant astigmatic direction for a first setting of the objective lens; (b). making an image, with the first astigmatic beam, of a specimen in the particle-optical device; (c). determining the direction of the astigmatic smearing (blurring) in the image made in step (b); (d). providing a different setting to the objective lens; (e). forming a second astigmatic beam from the aforementioned beam, which second beam has an attendant astigmatic direction for the other setting of the objective lens; (f). making an image of the specimen with the second astigmatic beam; (g). determining the direction of the astigmatic smearing in the image made in step (f); (h). comparing the direction of the astigmatic smearing in the image made in step (b) with that of the image made in step (f). (i). repeating steps (d) to (h) in the case that the aforementioned directions are equal, and, in the case that the aforementioned directions are not equal, performing an interpolation process between the first setting of the objective lens and the last-obtained setting of the objective lens so as to determine that setting of the objective lens at which the beam of electrically charged particles attains its optimum focus.