Zoom lenses underwater (Ikelite 20D housing)

Latest in a series of newbie type questions...I've never used zoom lenses underwater before and am a bit confused now with my new Ikelite housing.

For my Tamron 28-75mm XRLD Di lens, the Ikelite website suggested the 5503.80 dome. But I've found that once it's installed, I can't zoom in all the way to 75mm, because the lens bumps into the dome at about 60mm. Did I order the wrong port? Or is it simply because no port can accomodate the entire focal range?

If I didn't order the wrong one, is this because the nodal point of a zoom lens changes with focal length setting, and thus it's difficult/impossible for any single dome to be compatible with the entire focal range of the zoom lens? I.e., is the recommended dome port simply the best possible compromise for any given zoom lens?

Which leads to the question: At what focal length will I not achieve sharp images? Can I zoom in until the lens hits the dome and still get reasonably sharp images?

Coming from a different angle, what would happen if I house my kit 18-55 lens behind a port that's longer than optimal (i.e. one for a 10-22, the 5503.50)?

It's all a little blurry right now...

Thanks!

Ben

This is definetly a question for Ike. Try and email to ike@ikelite.com.

In the meantime, there is a problem. Your lens should not be able to hit the port, period. Since it does, the assumption must be that you have the wrong port or setup for the port. Not sure how Ike deals with extensions for ports. If each port is specifice to a lens, then you have the wrong one. If the portws use extensions, then yours is most likely too short.

Is it also possible that the Tamaron lens is a slightly different model thatn the one speced on the Ikelite site?

Food for thought only, as I am not an Ike user... try the email, as Ike has a reputation for the best serice and support in the business.

Ike came back with some quick feedback...the lens is probably a different one, but since I'm unlikely to be shooting at 75mm anyways, it should be ok.

Thanks Rich and Ike for the education...

Ben