The Haida Square Filter holder allows you to use 100mm square filters as opposed to traditional screw-type circular filters. These filters are compact and easy to store in the side pockets of your camera bag, so you can bring more to every shoot. The holder has 3-slots so filters can be stacked to create a stronger effect (especially when used with neutral density filters). The HD2500 fits lenses from 77mm-82mm with the use of Haida's metal adapter ring (sold separately). Square filters are also available in a variety of types, including Circular Polarising and Neutral Density.