An electron beam apparatus focusses an electron beam onto a specimen by means of an objective magnetic lens. In order to detect changes in the height of the specimen, a laser light beam from a laser source is incident on the specimen and the reflected laser beam is detected by a light detector. Any change in the height of the specimen changes the path of the laser beam to the detector. Therefore, by monitoring the detector, the focussing of the electron beam on the specimen can be controlled by varying the current to an excitation coil of the objective magnetic lens or by moving the specimen via a mounting stage. At least one of the pole pieces of the objective lens is on the opposite side of the path of the laser beam to the source of the electron beam, so that the objective magnetic lens may be close to the specimen, permitting a short focal length. Thus, the laser beam may pass between the pole pieces. An optical microscope may also be provided to permit the specimen to be viewed. The viewing path of the optical microscope extends through an opening in one or both of the pole pieces of the objective magnetic lens.