From the Manufacturer
The Canon PL-C polarizing filter enhances picture quality by blocking harmful reflected light. Use it to reduce polarized light reflections from glass and water surfaces or to improve color saturation. These effects can be seen through the viewfinder and changed by rotating the filter. The filter factor varies according to how the filter is rotated and its orientation to the sun. Filter factor is between 2.3 and 2.8 (approximately 1.3 stops).
This 52mm drop-in circular polarizing filter is specifically for the following IS (image stabilizer) super telephoto lens series: EF 300/2.8L IS USM, EF 400/2.8L IS USM, EF 500/4L IS USM and EF 600/4L IS USM. It can be rotated from the outside without removing it from the lens for precise control.
Product Description
Compatible with: EF 300mm f/2.8L, EF 400mm f/4 DO IS, EF 400mm f/2.8L IS, EF 500mm f/4L IS, EF 600mm f/4L IS