This Sony Drop-In Circular Polarizing Filter helps to reduce reflections and glare by filtering out light that has become polarized due to reflection from a non-metallic surface. It is made specifically to fit within the accessory filter slot of the Sony FE 400mm f/2.8 GM OSS Lens, whose front diameter is too large to accommodate a screw-on filter. With that lens, the effect of this filter occurs behind the main glass elements instead of in front of them.