Do dome ports increase minimum focus distance ?

[calypsonick]

I am having problems getting a focus lock with my 14-45mm lens in its Ikelite dome port. On the surface, the lens focuses without a problem at about 8" from the subject.

Underwater and behind the port, I find myself having to back way up to like 24" to get the focus to lock on. Is this just a normal optics thing or do I have other issues ?

Any advice appreciated.

Nick

 

[Gudge]

Underwater, dome ports act as an extra lens element that creates a virtual image that is smaller and closer to the camera than the subject really is. If the minimum focus distance of your lens is further than the virtual image is you won't be able to focus on it. To get around this problem you have to add a dioptre to the front of your lens (not all lenses will need one but yours obviously does). Your housing manufacturer should have a list of what dioptre your need (if any) for various lenses behind their dome ports. Failing this, in general you will need a +2 dioptre behind an 8" port and a +4 dioptre behind a 6" port.

Here's the Ikelite listing for Olympus:

http://www.ikelite.com/web_pages/2port_olympus.html

Looks like the 14-45 requires a +4 dioptre according to the first note under the table.

 

[calypsonick]

Thanks Gudge, it is now coming back to me. Off to search for some diopters......