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.

The light from the sun naturally becomes partially polarized due to reflecting off electrons in air molecules, causing the light to scatter into what appears as haze. A polarizing filter arranges, and filters, this directionally polarized light perpendicularly to the reflected light, allowing for the absorption of much of this light. This results in a noticeable reduction of glare and reflections from non-metallic surfaces and an increase in the saturation of skies and foliage.

A circular polarizer only differs from a linear polarizer in that it contains a quarter wave plane in addition to the linear polarizing filter. The addition of this quarter wave plane is to circularly polarize the linearly polarized light for the use of auto focus and exposure functions. Due to how the light is broken down and filtered with a linear polarizer, auto focus and exposure accuracy become skewed during use. When the light becomes circularly polarized, it permits the use of these functions.

This filter is constructed from optical glass for clarity and includes an integrated locking mechanism to secure it within the compatible lens, as well as an external dial to allow rotation of the filter glass for control of the polarization effect.