def _get_zoom_level(zoom, process):
    """Determine zoom levels."""
    if zoom is None:
        return reversed(process.config.zoom_levels)
    if isinstance(zoom, int):
        return [zoom]
    elif len(zoom) == 2:
        return reversed(range(min(zoom), max(zoom)+1))
    elif len(zoom) == 1:
        return zoom
Determine zoom levels.