Finally there is a way to stack a filter without having to remove the lens cover. FILTER CAP is held in place by the filter of your choice. As it's positioned behind the filter, it enables other filters to be stacked on top without causing any vignetting. FILTER CAP effectively blocks out dust and moisture. It also provides impact protection to the rim of the filter and lens which is often a common problem amongst avid photographers. This is truly a unique product.