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



William Graham wrote:

> "Prometheus" wrote in message

> >
> > Of course there is no water inside the lens (or should not be) however
> > there is water against the front of the lens and if a ray of light
> > reaching that front surface is not perpendicular to it then the angle of
> > refraction on entering the lens will be different between air and water.
> > Refraction can be considered as group delay across the interface, whereas
> > chromatic aberration is a result of phase delay, the former modifies focal
> > distance, the latter chromatic aberration.
> >
> Light travels about 75% as fast in water as it does in air, so any lens
> designed for air use will have an incorrect focus scale when used
> underwater....Light from infinity will focus behind the film plane when the
> lens is used underwater. but this effect will go away if water is prevented
> from contacting the front lens element by either a filter or an underwater
> housing, which would be necessary in any case if the lens was not designed
> for underwater use in the first place, so I wouldn't worry about it. Unless
> I was doing (and being paid for) professional underwater work, I would just
> buy a Nikonos with a decent 35 mm lens, and use that for my underwater
> pics....I wouldn't trust my F5 in one of those plastic underwater housings
> when you can buy a Nikonos with a good lens for under $500.........

Honestly, I thought you guys are all joking.
To sumurise theory of relativity, the speed of light is the universal
constant, it never changes, through vacuum, air or water. It refracts,
just no speed bumps...

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