The housing is sold separate from the ports (the front port). It comes without any port and those are added according to the lenses you use.
Indeed, you generally need a different port / extension combination for each lens, but many lenses work under the same dome port, and only require a different extension which is cheaper than a new port.
Ikelite offers either a modular combination – 8″ Dome + extensions in various sizes, or a fixed port option, which is just a smaller dome port and no extensions required.
The modular option is better if you plan to use several different lenses. The fixed option is more affordable.
The larger dome also improves optical quality, corner sharpness and makes it easier to shoot over / under shots (split shots).
Ran is a professional photographer for over 14 years. His passion for scuba diving and photography has pushed him to combine his profession and hobby and become a professional underwater photographer. Teaching is one of his greatest passions and over the years he has shared his experience with many divers and aspiring photographers. Along with his wife Danielle, an experienced Scuba Instructor, they have founded Dive and More, leading dive trips and UW photo workshops all over the world. Ran is also an electrical engineer and an avid internet marketing specialist. Visit his personal portfolio at www.ranmorphoto.com.