For underwater photography, the light must be getting bent differently than it would if the camera was above water due to the refractive index of water to glass. So, if I want to give a clue to tracking software, would it be a longer CG lens or shorter? Is there an exact formula, or even a rough one?
Cheers
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matt t
[ Editor ]
First you should find out if they were using a flat port or dome port.
http://www.camerasunderwater.info/optics/ports.html
If they were using a flat port then a rough chart guide can be found here along with the maths. Roughly a standard in air lens of 50mm would have the same field of view (around 90degrees) as a 35mm lens using a flat port underwater.
click 'angle of coverage' link on the webpage above
and if they were using a dome port you can assume the lens focal length should be about the same as used in air.
Please note these charts are for 35mm stills format.
You get a lot of chromatic aberration when shooting underwater footage so best to track only the green channel of the footage to get an average.
Cheers,
Matt
freds
[ Editor ]
Hi Jules, that's an interesting problem!
I found a thread that seems like it is trying to solve that problem, http://www.physicsforums.com/showthread.php?t=66525
I can't verify their conclusion though myself. :)
I wonder how many times you need to refract the ray? Because I suppose that there might be a housing on the camera so that it would be something like,
Imageplane -> Lens -> AirInHousing > Glass/Plexi Wall -> Water.
If you want to go that far. And how deep are you? Will the pressure press in on the housing glass making it convex? Though maybe that's just my £5.99 goggles and not high quality camera housing. :D
cheers fred
chris_77
[ Editor ]
I don't know this for sure, but....
think about it, WATER, which has a higher refraction index than AIR, but lower than LENS GLASS. It stands to reason the light would be bent LESS by the difference between WATER and GLASS than AIR and GLASS. Therefore the angle of view would be narrower. I'm just using my intuition here, and haven't provent it with math, but it seems to makes sense to me... - LONGER LENS sounds right - .... now I should go read that link! :)
And after reading their post - my conclusion and logic follows theirs.
It has to do with the RATIO OF THE INDICES OF REFRACTION OF THE MEDIUMS - less difference = less bending of light. less bending of light = straighter field of view...
easier to think of it in reverse... by tracing the frustum OUT of the lens... less bent in the water than in air.
ian_68
Hi Julian
Based on my PADI diving book. When under water looking through a flat piece of glass(goggles/cameraBox), objects appear about 33 percent larger than in air. So the basic idea is a 100mm lens will behave like a 133mm lens when placed in water. Having not done any underwater tracking, I can't say this will work, but might give you a starting point for adjusting lens info for tracking.
There are other factors as well. Because water is very good at holding particles, this can have great effects on how far you can see, as well as diffusing focus.
The last factor which makes water different from air is how light colours are absorbed. As White light travels through water, the water absorbs colours one by one: first red, followed by orange and yellow, with green and blue last.
my two pence, Ian