def twolens_efl(efl1, efl2, separation):
    """Use thick lens equations to compute the focal length for two elements separated by some distance.

    Parameters
    ----------
    efl1 : `float`
        EFL of the first lens
    efl2 : `float`
        EFL of the second lens

    separation : `float`
        separation of the two lenses

    Returns
    -------
    `float`
        focal length of the two lens system

    """
    phi1, phi2, t = 1 / efl1, 1 / efl2, separation
    phi_tot = phi1 + phi2 - t * phi1 * phi2
    return 1 / phi_tot