The Kase K9 holder system has an innovative solution to the problem of using circular polarisers with slot-in systems. Rather than being mounted as the last filter, making it large and allowing the possibility of light being reflected back into the lens. The Kase solution is to mount the polariser at the back of the system, allowing you to have up to three Neutral Density or Grad filters in front of the polariser.
The polariser is mounted to a geared lens adaptor ring, that is then turned simply by using a gear on the holder when attached to your camera. To mount the polariser, it’s simply a case of placing it into the geared ring magnetically, removing the hassle of trying to screw a polariser into place.