The PL-C 52WII 52mm Drop-In Circular Polarizing Filter from Canon is designed for use with the EF 300mm f/2.8L IS II USM and EF 400mm f/2.8L IS II USM lenses, which accept rear-mounted 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.

Circular polarizers help to reduce reflections and glare by filtering out light that has become polarized due to reflection from a non-metallic surface. This results in a noticeable increase in the saturation of skies and foliage, as well as clearer imagery when photographing in hazy conditions.