You're looking at a B+W 72mm Slim Circular Polarizer With Lens Cap - New

Highly efficient standard circular polarizing filter for all cameras with beam splitters in the light paths of their TTL exposure meter and with autofocus lenses. Circular polarization has the same pictorial effect as linear polarization, but allows for proper exposure metering and/or autofocus distance settings.

Circular Polarizers increase color saturation and reduce reflections. The neutral gray color and plane parallel polarizer material guarantee optimal image results. High-quality optical glass ensures excellent pictorial quality. The filter factor varies according to how the filter is positioned in relation to the sun. Exposure compensation is about two f-stops.

Linear and circular polarizers both consist of a linear polarizer foil but differ in their construction in the following way. Modern DSLR cameras have a beam-splitting prism that sends part of the incoming light to the meter and part to the viewfinder. The effect is that the light entering the meter is partially polarized by the beam-splitter. A linear polarizer placed on the lens of such a system will act as a second polarizer and block light to the meter by a degree dependent on the angle between the prism and the polarizer on the lens. The result is incorrect exposure/aperture values from the meter. The circular polarizer circumvents this problem through the addition of a 1/4-wave retarder, or delay foil. This ensures that the linearly polarized light is changed into a rotation that appears unpolarized to the meter, resulting in proper exposure/aperture readings.

Features:

• Removes Reflections And Increases Color Saturation Without Affecting The Overall Color Balance
• Brass Is Used To Make Most Of The Rings For The Filters And Adapter Rings. Aluminum Is Used For The Rigid Lens Hoods Like The telephoto Metal Lens Shades. For The Rubber Lens Hoods A Special Rubber Mixture Is Used
• Filter Factor: Between 2.3 And 2.8 (Approx. +1.3 Stops)
• For brilliant color reproduction, best contrast and sharpness Heliopan offers the highest quality optical glass from the German Schott Group.
• High quality hard coating on each filter side to reduce stray light, to maintain high contrast and avoid flare
• Slim mount does not vignette with 21mm lenses.
• Minimal reflections because of matte black anodized brass filter rims which will not bind when threaded in combination with other metals.

Circular polarizers are awesome.  The main reason you'd want one is to reduce reflections on surfaces.  You'll see it on water, on hoods, streets, etc.  Anything that will reflect light will polarize the light and the polarizer will remove that glare.  This will help to saturate a blue sky and help to give definition to clouds as well.  Heliopan is one of the highest quality filters you can buy.  

The main reason you'd want this particular filter is for really wide angle shots where you want a polarizer but you also need to make sure that the filter ring doesn't show up in your pictures.  This is is about 6mm thick while standard filters with a front thread are around 8mm.  They say you can get to about 21mm wide without vignette

I've had this filter in my camera bag for years and I had other ones that I used regularly.  It's time to let it go to someone that can use it.  It's been used and it looks like it may have taken a hit at some point. See pictures for chip in edge of the glass.  It's small enough that it shouldn't affect pictures in a noticeable way.

This auction includes the Heliopan Slim Circular Polarizer and a storage case.  

Shipping will be by USPS.

Good Luck!