Abstract: Exposure measuring system for attachment cameras to microscopes provided with a focusing telescope which is provided with a transverse slot for the selective insertion of two sliders, one of which is provided at the approximate slider center with a photodetector for the exposure measuring system while the other consists of a transparent plate which bears a marking of circular shape or the like also approximately at the center of the slider and corresponding in size to that of the photodetector. The slot possibly serves only for the introduction of a single slider bearing both the photodetector and the marking a given distance apart and is correspondingly displaceable between two positions. The guide path (10) of the slider(s) (9) is displaceable in the plane of the slider and means (16) actuated from outside the focusing telescope are provided to cause slider displacement.