Sigma TS-121 Tripod Socket: Designed for Sigma 70-200mm F2.8 DG OS HSM lens. The Sigma TS-121 Tripod Socket is a spare or replacement tripod foot for the one originally included with the lens. Constructed from magnesium alloy, this foot is Arca-type compatible for direct use on select tripod heads and also features a 1/4"-20 socket for attaching to an additional quick release plate and any tripod.