A special extension Ikelite Port Extension 1.75" with Manual Focus Control is specifically designed for use with Nikon 105mm VR Macro and Canon 100mm USM Macro lenses. Manual focus is adjusted with a large, easy-to-turn knob on the side of the extension. Extension can be used with the Nikon 105mm VR Macro lens and either 5510.16 or 5510.22 Lens Extensions. Canon 100mm USM lenses require the 5510.16 Lens Extension. The Focus Extension can also be used in place of Lens Spacer 1.75 in in most instances. The Focus Extension should no be used with the bayonet-style lens extensions 5510.10 or 5510.11.