Flat ports are recommended for standard, macro and telephoto lenses. Due to refraction, flat ports reduce the angle of coverage of a lens and magnify the image. This makes the flat port ideal when using standard, macro or zoom lenses to shoot small subjects.
This port is designed specifically to provide manual focus with the Canon 100mm f/2.8 EF USM Macro lens. The newer Canon 100mm f/2.8L EF IS USM Macro lens is not operable in this port. Use Focus Port 5508.46 instead.