def mag_to_object_dist(efl, mag):
    """Compute the object distance for a given focal length and magnification.

    Parameters
    ----------
    efl : `float`
        focal length of the lens
    mag : `float`
        signed magnification

    Returns
    -------
    `float`
        object distance

    """
    return efl * (1 / mag + 1)
