The main function of a circular polarizing filter (or CPL) is to reduce polarized light. The best way of understanding this is to look through a pair of polarized sunglasses. It works by cutting out the light reflected from hard or reflective surfaces.
Reflected light like this can be blocked because all this reflected light is bouncing into the lens from the same direction which means a finely striped filter, when lined up correctly can block only those rays.
Because this effect can only be achieved by lining up the filter correctly. The element is free to spin on the end of the lens once screwed in. The screw thread is a standard universal fitting, so as long as you get the right diameter you can be assured a secure fit.
When looking through the lens, as you rotate the CPL filter you will see the effect change through the view finder. It is particularly noticeable when looking at blue skies or strong greens such as plants, creating richer colours and allowing the camera to capture a greater range of information.
Unfortunately, as the rotation of the filter effects the picture, lenses which rotate as they focus will change the effect. This means you must frame and focus your image before finally fine tuning your filter.
With any additional filter you are letting less light into your lens so you may have to over expose a stop or half a stop to get the desired effect. Thankfully the quality of Nikon glass is excellent and it's positive effect far outweighs this inconvenience.
The build quality is good and my filter is still in very good shape after a year . The bezel still rotates easily but stays still once set. The branding along the side is clear so it is easy to locate in the kit bag.
For me this a great bit of kit, and I still use it regularly. If your new to SLR its a great way to richer shots with a little effort.