Dee once bubbled...
What's the reason for a dome port? I think it has something to do with the curve you see in some wide angle pictures...don't know the technical speak for it.
So could someone explain it in plain english for me? I've used wide angle lenses above and below the water and never had that curve.
Not all lenses create the curve effect (distortion) you speak of, and it is not directly caused by a domed port. Actually a domed port is used to correct the distortion on WA lenses caused by a flat port. The photo I used above shows a slight curve in the waterline, but this was caused by the slight surface chop.
here is excerpt from an artical written by Franklin J. Viola about the "BENEFITS OF AN AQUATICA HOUSED SLR", it explains how a domed port works;
SELECTING A PORT
The next step after choosing an SLR lens is determining which port to use. The angle of coverage for a given lens is determined by the port you place between it and the aquatic optic ("water"). Due to the refractive properties of water, "flat" ports reduce a lens' angle of coverage by approximately a third. Alternately, "dome" ports correct for the refractive property of water so the lens' coverage is the same as on land. How does a dome do this? Underwater a dome port functions as an additional lens which creates an apparent or virtual image of the actual image. The apparent image is located at a distance approximately twice the diameter of the dome from the camera's film plane, even though the actual subject may be several feet away. An SLR lens behind a six inch dome must be able to focus down to twelve inches, and respectively, to sixteen inches for an eight inch diameter dome. If it can not, then a "close-up diopter" (a magnifying lens) must be added to the SLR lens to reduce its minimum focus distance. Dome ports also increase color saturation and definition of underwater images. Traditionally, flat ports (which actually increase the magnification power of a lens underwater) are used for shooting macro images. Conversely, dome ports are used with wide-angle lenses for underwater scenic shots. This fundamental thinking also applies to Nikonos V lenses where the front optic of the 35mm and 28mm lenses are flat, while that of the 20mm and 15mm are curved.
There is some other good information in this artical, you can find it here:
http://www.aquatica.ca/news.html
This is a lost link and you can no longer find it on Aquatica's home page, but it still works.
