The Optolite Port:

The SEAPRO OPTOLITE PORT is exclusive to Greenaway Marine.
This underwater optical corrector enables the video camera to be used above water, below water, even half in and half out of water.
The depth of focus is outstanding, being from a few millimetres in front of the Optolite port to infinity in most cases. You can zoom in air or underwater, it increases edge definition by producing a sharper image.

HOW DO OPTOLITE PORTS,  FLAT PORTS AND, DOMES COMPARE?

Optolite Port: The port has a concave inner surface and a flat front. You have one focus setting for in ..Air and Underwater, it can therefore be used half in.. half out. This is the ultimate in Optical Flexibility. All shots whether underwater....Half in or Half out...or on the surface....all Pin sharp. Focusing is just a thing of the past. The optolite port corrects the loss of viewing angle, when using a flat port, by increasing the angle of view then when you enter the water the angle of view returns to the original angle. Call us if you need this explained. 01793 814993.

This is best corrector for underwater use, with its other advantages.
  
The Optolite Port has vast depth of focus, you are able to produce wide angle close ups even with the subject millimeters from the Port.

FLAT PORT You need to focus for all shots, modern auto focus video cameras do this very well.
SEAPRO use flat ports for this reason.
Your image will fall off and have barrel distortion at very wide angle lens settings, but underwater you rarely have straight lines to show this distortion, plus most of the time the depth of field hides the fall off. If you use high quality wide angle lenses the problem becomes more noticeable.
You do, however, have your angle of view reduced by 25% when using a flat port and you have no depth of focus.
You can produce a half in half out shot, but the water air interface will be out of focus, producing an unsightly line across your image.

DOME PORT The main use of the dome port is to restore the angle of view of the lens when underwater. It acts as a clear window when on the surface. However, when underwater it produces an apparent image that you must focus on to produce a sharp picture. This means you must focus at approximately 12" underwater. If you move the housing out of water in to the air, the camera will still be focused at the apparent image at approximately 12". This will mean the in air picture will be out of focus.
 Some housing manufacturers fit a +4 diopter close up lens between the dome and the wide angle lens, this produces a good image underwater but will always be out of focus in air.
 To produce a half in and half out shot you must use a 8" diameter dome. As most video camera housings are only 7" in diameter this is impractical and expensive.
 Dome ports also have two further drawbacks, they are exposed, protruding from the main housing body, just waiting to be scratched or knocked and they are also very susceptible to flare when looking towards the surface.

The Optolite port overcomes all these problems:

If you require the highest optical quality the SEAPRO Optolite Port is the one to go for, the flat port is the next best compromise.
Optolite is a specially formulated plastic originally produced as a substitute for lenses in high quality optical instruments. It is now being used by Greenaway Marine exclusively in the production of the Optolite Corrected Port. The units are Ground and polished then mounted into the Anodised Aluminium Holder.
Ports can be interchangeable if required, at extra cost. Most are fitted as a permanent unit.