def defocus_to_image_displacement(W020, fno, wavelength=None):
    """Compute image displacment from wavefront defocus expressed in waves 0-P to.

    Parameters
    ----------
    W020 : `float` or `numpy.ndarray`
        wavefront defocus, units of waves if wavelength != None, else units of length
    fno : `float`
        f/# of the lens or system
    wavelength : `float`, optional
        wavelength of light, if None W020 takes units of length

    Returns
    -------
    `float`
        image displacement.  Motion of image in um caused by defocus OPD

    """
    if wavelength is not None:
        return 8 * fno**2 * wavelength * W020
    else:
        return 8 * fno**2 * W020