Years ago virtually all microscope objectives were designed to produce an image of the specimen 160 mm behind the objective. This figure shows a laser ablation system based on this sort of microscope. Lens y focuses the beam to a point within the image, so that it will focus to a corresponding point within the specimen. Lens y can be moved along the beam axis to adjust the focus of the laser so that it corresponds to the image you see. If it is moved toward the laser, the focus will move upward in the specimen.
Many microscopes nowadays use infinity-corrected objectives.
The laser light cannot be focused to a point in the image, because no image is formed. If you don't know whether your microscope forms an image, you can test it. It is important that laser light enter the specimen from the widest possible range of angles, which means that beam diameter must be at least large enough to illuminate the entire objective. Lenses w and x form a simple Galilean telescope that allows the beam diameter to be adjusted. If lens x is moved slightly towards the laser, the beam will diverge slightly as it leaves the telescope, so that its diameter will be larger at lens y and the objective.