def semi_angle_limit(Lambda, C_3=1):
    """
    Convergence semi-angle if the probe size is diffraction limited.
    Params:
    Lambda: electron wavelength in Å
    C_3: spherical aberration constant in mm, default 1.
    Returns:
    Convergence semi-angle in mrad.
    """
    C_3 *= 1.e7
    return 1.51 * C_3 ** (-1. / 4) * Lambda ** (1. / 4) * 1e3