There are two types of Polariser, circular and linear. These names bear no relation to the physical shape of the filter itself but refer instead to the way the light waves are transmitted through the filter to achieve the polarising effect.
The SW150 System features its own circular polarising filter. If used alone, the polariser can be placed into the filter holder and the holder rotated until the desired effect is achieved. If used with a grad filter, the grad should be positioned first, then the polariser added – you will have no rotation possible without turning the grad, but will have a choice of two axis of polarisation, achievable by turning the filter through 90 degrees.
Made by UK company LEE Filters; their reputation has been built on the fact that every filter that leaves the factory has been individually handmade and inspected by one of their technicians, who ensure it meets their exacting standards. "Anything that will compromise image quality, is simply not an option."