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



In article <5q-dneHKmvva3dDZnZ2dnUVZ_tOdnZ2d@comcast.com>, William
Graham writes
>
>"cjcampbell" wrote in message
>news:1145864866.698218.269710@e56g2000cwe.googlegroups.com...
>>
>> Prometheus wrote:
>>> In article <1145858451.759527.73370@u72g2000cwu.googlegroups.com>,
>>> cjcampbell writes
>>> >
>>> >Dave Hillstrom wrote:
>>> >> You can't use regular lenses underwater, that's why you always see
>>> >> special cameras on ocean documentaries and such. People think it's
>>> >> because they need to be waterproof but the real reason is the speed of
>>> >> light underwater is different than in air and it makes the light bend
>>> >> at different angles through the lenses.
>>> >> Einstein proved it.
>>> >
>>> >So, I suppose that is also the reason you need different lenses to take
>>> >telephoto pictures than the lenses you would use for wide angle. Light
>>> >from different distances travels at different speeds? No doubt this is
>>> >due to the fact that light traveling from farther away has to go
>>> >through more haze, so it passes through more water on the way to the
>>> >camera. It would explain depth of field, too. A lens set at a
>>> >particular focal length can only bend light traveling at a particular
>>> >speed at the correct angle. Light that is going too fast or too slow
>>> >would, of course, bend at the wrong angle and thus the foreground and
>>> >background are out of focus. That being the case, you would also need
>>> >special lenses in space, where light does not travel through any water
>>> >at all.
>>> >
>>> >Or maybe it is just wrong to encourage this nut.
>>>
>>> No, it is you who is wrong. Light DOES travel at a lower speed through
>>> water than air The refraction at the water/glass interface of the front
>>> element when immersed in water is very different to the air/glass
>>> interface when in air. The effect of water vapour in air is much, much
>>> less; although droplets will cause random refractions and make objects
>>> blurred.
>>
>> Okay, seriously, I know that the speed of light changes slightly as it
>> travels through water, glass, and just about anything else. That said,
>> the lenses used in underwater photography work perfectly well on dry
>> ground, in space, or just about anywhere else.
>>
>> I should have known that attempting any humor at all on Usenet is
>> dangerous...
>>
>
>Yes. - There is no water inside the lens, so obviously, it operates
>underwater just as it would in air. In general, the only difference between
>regular lenses and those designed for underwater use are the seals that
>prevent water from entering the lens and damaging it internally. Optically
>both types of lenses are the same. Lenses themselves depend for their
>operation on there being a difference between the speed of light thru the
>glass and its speed in air, or whatever medium is outside the glass. How
>fast the light gets from the subject to the front element of the lens is of
>little significance.....

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.

--
Ian G8ILZ

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

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