This is the Heliopan 40.5mm Circular Polarizing Filter is used to select which light rays enter your camera lens. They can remove unwanted reflections from non-metallic surfaces such as water or glass and also saturate colors providing better contrast. The effect 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. One of the "wow" factors of this filter is the index marking on the edge of the filter ring. This allows you to hold the filter up to your eye, choose a point that you like for polarization, replace the filter on the camera and simply rotate the ring to the chosen mark. Heliopan Slim Circular Polarizers have no front threads. This filter is the best way to make the clouds stand out, increase or decrease the saturation of the ocean or increase or decrease the reflection off the water. You can also use this filter when shooting into glass or windows since it will, again, either increase or decrease reflections.