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After some discussions in other threads about underwater synths of reefs, and the cool factor of having a synth that would bridge the above world and the below world so you could start out on the beach and wind up underwater, I started thinking through how you could pull off that above-to-below transition. Not gonna be easy, but I think it's doable.
But by far one of the most useful tricks would be to have a camera lowered into the water by degrees, taking pictures as it goes. Even more useful would be to do 360 degree panorama sequences from at least three spots, preferably more, so that there's plenty of parallax to tie the above and below frames together.
This is where it gets interesting, though: The index of refraction of air is a lot lower than the index of refraction of sea water, or even fresh water. Since Photosynth can synth pictures from different cameras, I'm guessing it would have no problem synthing air shots and water shots into a single frame of reference.
But what about the transitional frames? The ones where there's air above and water below? Any chance that would work? Or is it going to give Photosynth fits since the underwater scene will look like it was taken with a different lens geometry?
I'm pretty stoked about trying this, so eventually I hope to be able to say one way or the other. But if anyone has any ideas before I go off and build this enclosure, I'd be more than willing to listen to reason.
It seems to me that the most likely to succeed method would be to carry some super synthy object out that is highly visible above water and extends down into shallow water by, say, ten feet which won't be distorted too much by the water's movement.
Aim at it and hopefully come equipped with some way to move in on your visual marker from above the water by say fifteen feet (maybe mount your camera on your mast) and take a sequence centred on the lower end of your marker, taking extra shots while passing through the water's surface.
I do love shots of simultaneously above|below water, but with a typical camera's lens size, there are limited numbers of these shots you can take with any variation. I think a camera descending with the lens pointing down at somewhere in the neighbourhood of 45 degrees stands the greatest chance of linking.
This is just speculation on my part though. Nothing beats actually shooting a location. :)
Here's my thought on how I'd like to do this:
I'm glad you pointed out the size of the lens. With an underwater housing, I'm not convinced you could really get many "split the scene" style shots. The typical chop in the ocean is just too rough, and shots would either tend to be all the way above the water or all the way below.
Rather than use an underwater housing for the transition, I was thinking of making a box about 2'x2' with one side either Lexan or tempered glass. Box goes in the water (weighted down by cinder blocks, rocks, sand, whatever) and the camera is slowly lowered through the transition from air to water with, as you said, an object in the frame that extends from the water to the air. The hope here is that there will be a number of images in the set with a clear split in them, showing above and below.
I don't think this approach is without problems, though. Glare off the glass is one. (No, I'm not planning to get a 2'x6' pane of tempered glass that's been anti-reflection coated!) So careful shading of the glass and camera will be important. Moisture on the inside would also be a killer since Photosynth may happily synth off the drops rather than the outside scene.
And from a more practical standpoint it's going to be ungainly to use and fairly expensive to build. In short it'd be a major pain in the rear. But if the synth worked, it would be COOL.
I've got a beach picked out to try this at. The beach at Wailea Bay on the Big Island has some really big rocks out in the water that span the air/water interface by a big margin. Even better, the rocks have lots of coral on them below water. Better still, it's about as photogenic a beach as you can find.
Now I just need the materials and the time to set it up.