The Filter Holder with WR Circular Polarizer RCP-11 Drop-In Filter from Sigma is compatible only with Sigma's 500mm f/4 DG OS HSM Sports Lens. The filter holder fits into the rear slot of the telephoto lens and locks into place. At the top of the filter holder there is a small wheel that allows you to turn the circular polarizer within the holder to get the desired effect. By using the included WR Circular Polarizer RCP-11 Drop-In Filter, which has a filter factor of 2 to 4x, you can increase your exposure by 1 to 2 stops