The Kase Magnetic Circular Adapter Ring 77mm is a spare or replacement, which fits popular magnetic circular filters and can be left mounted to your lens, so that you're ready to go at a moments notice. This is a spare or additional magnetic ring designed for use with Kase's magnetic circular filter system. This can also expand your kit bag so that you have an adapter for all your lenses, allowing for quick and easy switching between lenses/filters.