The lee sw150 mark ii adapter screws onto the end of the sigma 12 24mm f45 56 dg hsm ii lens to allow the use of the sw 150 mark ii filter holder not included available separately each adapter consists of a front ring a compression ring and a locking ring that have been specifically designed for each particular lens when fitted they form a collar that remains on the lens at all times not compatible with the older sigma 12 24mm lens