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.
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.