When the tripod collar of the Sigma 150-600mm f/5-6.3 DG OS HSM Contemporary lens is removed, the Protective Cover PT-11 by Sigma can take its place surrounding the lens barrel. This is a replacement item for the one originally included with the lens and serves to protect specific area of the lens barrel when the tripod collar is removed for a lighter-weight option when photographing handheld with the long telephoto zoom lens.