This Lens-Specific Flat Port is for the Sigma 70mm f/2.8 EX DG Macro Lens.
FLAT PORTS... For standard, macro and telephoto lenses. Due to refraction flat ports reduce the angle-of-coverage of a lens and at the same time enlarge the image. This makes the flat port ideal when using standard, macro or zoom lenses to shoot small subjects.
Note that if a port is long enough to accommodate the full length of a 1:1 macro lens, the lens may vignette when used at infinity. If access to the lens' full range is desired, then select a dome port instead.