The PL-C 52WII 52mm Drop-In Circular Polarizing Filter from Canon is designed for use with select super telephoto lenses that accept rear-mounted 52mm drop-in filters. The filter is rotated from a ring on the outside of the filter mount. You do not have to remove the filter to use it. The glass is secured within a strong metal frame that slips in and out of the rear mount with ease. The degree of polarization can be seen through the viewfinder of your camera as you rotate the filter.