The existence of a ring‐shaped constriction in the electron beam of a thin lens beta‐ray spectrometer is demonstrated experimentally by a photographic film method. Making use of a photographic mapping of the beam, an annular baffle is designed. Optimum relations between annular width, solid angle, and counter window diameter are investigated. A comparison of this type of baffle system with the original baffle system shows that at 2.5 percent resolution the spectrometer transmission may be increased by a factor of two by the use of an annular aperture in the region of beam constriction.