This particular lens is equipped with auto focus for best possible result. If you want to use a filter on the lens, then the filter size is 72 mm.
From Photospecialist
Sigma 16-28mm F/2.8 DG DN Contemporary Sony FE
The product is available from £670.46 at international stores.
OnBuy
Sigma 16-28mm F/2.8 DG DN Contemporary Lens (Sony E)