"Due to refraction flat ports reduce the angle-of-coverage of a lens and at the same time enlarge the image, ideal for shooting small subjects with a standard, macro or zoom lens. Note that if a port is long enough to accommodate the full length of a 1:1 macro lens, the lens may vignette at infinity when used behind aflat port. Note Port Body Not Included An appropriate Port Body is required to attach a Flat or Dome Assembly to the housing. Port Bodies are offered in a variety of lengths for use with different lenses. Please note that picture sharpness is very sensitive to port length when using a dome underwater, while the optics of a flat port underwater are very forgiving. We recommend selecting a port body based on your wide-angle or zoom lens. The #5510.16 and #5510.22 are the most common Port Bodies for popular wide-angle lenses. Both accept port extensions to accommodate longer macro and zoom lenses."