The nominal value of the effective camera length of an EM operating in the selected area mode is not sufficiently accurate for any calculations of lattice spacing. The actual value of camera length must be calibrated as the same accelerating voltage and objective lens setting by reference to a known substance with well defined diffraction spacings. The normal specimens are evaporated films on a 3.05 mm grid of aluminum or thallous chloride. The very small crystalline size yields ring patterns suitable for calibration purposes.