Perhaps more than any other filter, a circualr polarizer filter can add significant POP to your pictures. And a drop-in filter such as the Canon 52mm Drop-In Circular Polarizer Filter is the only way to add a circular polarizer filter to Canon's Super Telephoto Lenses including the Canon 300mm f/2.8 L IS II Lens, Canon 400mm f/2.8 L IS II Lens, Canon 500mm f/4 L IS II Lens and Canon 600mm f/4 L IS II Lens.
A simple pinch of the Drop-In filter latch opens and allows insertion/removal of the filter. A small wheel exposed at the top of the filter allows rotation adjustment of the Canon 52mm Drop-In Circular Polarizer Filter for specific lighting conditions.
Please read my B+W 77mm MRC Circular Polarizer Filter Review for more information about Circular Polarizing Filters. The Canon 52mm Drop-In Circular Polarizer Filter functions basically the same as the 77mm CP Filter.