def ij_tag_50838(nchannels):
    """ImageJ uses tag 50838 to indicate size of metadata elements (e.g., 768 bytes per ROI)
    Parameter `nchannels` must accurately describe the length of the display ranges and LUTs, 
    or the values imported into LUTs will be shifted in memory.
    :param nchannels: 
    :return:
    """
    info_block = (20,)  # summary of metadata fields
    display_block = (16 * nchannels,)  # display range block
    luts_block = (256 * 3,) * nchannels  #
    return info_block + display_block + luts_block