Penflex Circular Polarizers (CPL) are manufactured from Glass with Polarizing materials incorporated. The rings are metal, and feature a male thread which screws into the filter, and a female thread on front to accept lens hoods, other filters, or accessories. Once attached to a lens, the front ring of the polarizer rotates and may be adjusted for maximum effect. This is typically done by looking through the viewfinder while turning the front ring on the filter. Slim Design: Designed with a slim design metal ring. The thickness of the metal ring projects only 6.5 mm past the front of the lens. Knurled front edges provide a better grip. Total thickness of the filter is only 8.2 mm. A polarizing filter is placed in front of the camera lens in photography in order to darken skies, manage reflections, or suppress glare from the surface of lakes or the sea. Since reflections (and sky-light) tend to be at least partially linearly-polarized, a linear polarizer can be used to change the balance of the light in the photograph. The rotational orientation of the filter is adjusted for the preferred artistic effect.