Abstract: A retention device for use with a filter apparatus having a main frame and an intermediate frame member, and a plurality of parallel filter elements in a filter zone being spaced one from another at a selected distance and forming a filter screen known as a harp screen, is disclosed. The filter screen is mounted to the main frame. The retention device includes a clamp disposed at an intermediate portion of the filter elements for clamping at least a clamped portion of the filter elements to the intermediate frame member, with the arrangement being such that a neutral region is provided adjacent or within the clamped portion of the filter elements, the neutral region being such that the filter elements may be manipulated within that region, and their relative spacing varied, within that region, without affecting spacing between filter elements outside the neutral region.