I just bought a Fujinon 180mm F5.6 soft focus lens from Japan, it came with a yellow grid/strainer. This grid can be loosely inserted to the rear of front lens, but can not be sapped on like its larger version lens, 250mm F5.6 soft focus lens, does. Is the insertion of yellow grid on the rear of front lens the correct way to use on 180mm F5.6 lens? Thanks.