OWL is the World’s First Drop-In Filter Adapter That Lets You Swap Filters in a Flash
For telephoto lenses with huge front elements, screw-on filters aren't very practical or economical. Thus, for certain lenses, companies like Canon and Nikon offer drop-in filters that slide into a slot close to the rear element.
There's a new product that hopes to use this same style of filter for smaller camera-and-lens combos. Called OWL, it's the first drop-in filter adapter.