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




"Prometheus" wrote in message
news:Th8UsthqRVTEFwDz@newbrain.demon.co.uk...
> 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.
>
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.........



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