Abstract: A high sensitivity beam deflection sensing optical device, such as an atomic force microscope, including one or more of the following: specified means in the path of the incident beam for adjusting the size and/or power of the incident beam spot, means for moving the incident beam spot with movement of the object whereby to maintain the position of the spot on the object, and means for increasing the signal to noise ratio of the optical detector in which adjusted gains are applied to different segments of the optical detector.

Abstract: An atomic force microscope utilizing an optical system having a numerical aperture sufficient with the wavelength of light of an incident beam to form a spot on the cantilever having a size of 8 .mu.m or less in at least one dimension. An adjustable aperture can be utilized to control the size and/or shape of the incident beam spot on the cantilever. Portions of the incident beam and the beam reflected from the cantilever overlap and are directed so that the plane of focus of the incident beam is parallel to the plane of the cantilever. The incident and reflected light beams are separated by polarization using a beamsplitter in conjunction with a quarterwave plate. Focussing can be accomplished with a confocal viewing system coupled with a translatable focusing lens common to the optical system and viewing system.