I'm confused. What are differences between the following:
Dome Unit for UWL-100 ($389)
UFL-165AD Underwater Fisheye Conversion ($389)
UWL-100 Achromat Wide Conversion Lens, Type 1 and 2 ($329), which Type for G9?
(from http://www.inonamerica.com)
Which do you recommend? Do I need Dome Unit? Why?
I currently own two of the lenses on your list (UWL-100 & UFL-165AD), and I soon plan to purchase the Inon Dome Unit for UWL-100.
First you must understand that there are two lenses on your list, and one dome which converts the UWL-100 form a wide angle to a semi-fisheye super wide angle. Also, you need to understand that the UWL-100 and the UFL-165AD are for different base mounts and are not intended to be used on the same camera.
The UWL-100 actually comes in to different models (Type 1 & Type 2). The type 1 fits certain cameras and requires an adapter ring. The type 2 is for all other cameras with 67mm threads directly on the housing (no adapter required).
The UWL-100 is a wide angle lens that will give you approximately 100 degrees maximum view angle. It's a great lens, because it will allow you to get real close to your subject, yet still cover a wide area. If you would like to get even closer and cover a wider area, than you can add the dome unit to it and it will give you in the area of 130 degrees maximum view angle. However, when using a fisheye lens, the angle can get so wide, that sometime when shooting close objects the outside perimeter of the picture can look warped.
Ths UFL-165AD lens is a fisheye lens that will allow you to focus directly up to the lens and back to infinity, but it only fits mount bases that use the AD Mount. I have it, because I also own a Sony Camera with a Sony Housing that uses the AD Mount Base. This lens will give approximately 165 degrees of viewing angle underwater, however, this lens is not really usable with the G9, because the diameter of the lens is too small and not really compatible with the G9.
I have not really looked at what requirements the Ikelite housing requires to attach lenses, but I'm pretty sure it would either use the UWl-100 or Ikelites Proprietery wide angle lens.