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Subject: Re: You use different lenses underwater because the speed of light is different



Celcius wrote:
> Hi Dave!
>
> Please explain why on my Nikonos camera, I had a 35mm lens which could
> photograph both underwater and and land? Just curious. There was also a
> 28mm for underwater only as well as others. The only other factors that
> I know of is that underwater, things seem closer than on land and warm
> colours disappear as you go deeper.
>
> Marcel
>

Your Nikonos camera has one additional lens in the system which you have
probably ignored: the air-water interface (well actually
air-glass-water interface, but the glass is thin and close enough to the
index of refraction of water that it can be neglected for gross
calculations).

This interface is just like the glass-air interface in a regular lens
except that it is not curved. As it is not curved, it will not tend to
focus the light to a single focal point like a regular lens, but it will
still refract the rays crossing the boundary. The result is the well
known "objects appear 4/3 closer" business. The focal point of the rest
of the system remains unchanged: the film plane is the same even after
you remove the water and take pictures in air.

The result is that you should be able to take decent pictures in or out
of the water, but your field of view will be much greater out of the
water than in it.

There are however a few tradeoffs. The flat piece of glass absorbs some
of the light of course, and also introduces some distortion. For
instance, every ray does not pass through the same thickness of glass
due to the angles, so light coming in at angles will be shifted as well
as refracted, and the extra lens will also increase the chromatic
aberration.

You should be able to counter these somewhat, but it's a choice whether
optimize for water, optimize for air, or optimize for average overall
performance and no matter what, your camera will have one more lens than
a similarly optimized purpose built dry or wet camera. This means that
a dedicated camera designed for either air OR water will be able to
outperform a camera designed with both purposes in mind.

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