def METHOD_NAME(deltaf_hz, f_0):
    """Converts (ex: FWHM) from ΔHz to Δnm.
    Parameters
    ----------
    deltaf_hz: Hz
        frequency broadening
    nu_0: Hz
        center frequency
    """
    return hz2nm(f_0 - deltaf_hz / 2) - hz2nm(f_0 + deltaf_hz / 2)