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



In article , Joseph Meehan
wrote:

> Funny thing. You are about half right, almost.
>
> Yea, light travels at different speeds. It even is different in glass,
> or different kinds of glass. That has been known for a long time.
>
> It does means that the focusing scale marked on the lens is not going to
> be correct, but it does not mean the lens needs to be changed.


Consider all the following:

1) The extreme front element on most all multi-element lenses has a
curved front surface, curved to bend light, of course.

2) How much that front element bends light depends on what the very
front curved surface of the element is in contact with. It is usually
in contact with air, however it could be used in contact with water,
oil, glycerin, alcohol, or many other substances.

3) The overall correction of optical defects depends partly on that
front element bending light in such-and-such a manner.




The best illustration of what this means is when the front element of a
microscope lens is not properly used.

Namely, a 1,000x "oil immersion lens" where the glass of the front
element is designed to be used in contact with oil.

Such a lens, if improperly used in air, has its optics so messed up
that it is useless.

The microscope lens can still rock through the focus point, but so much
chromatic abberation creeps in that the lens is unusable.


Interestingly, in the situation where a regular camera is used inside a
glass box filled with air, the lens optics do _not_ get degraded
much, because the outer front surface of the curved lens is in contact
with air, like it was designed for - - - but now the photograper has to
worry about whether the glass front of the box is optically flat.


Most photographers do not notice the decrease in sharpness when a
"water-resistant" camera is used underwater, but tests for sharpness
might reveal a decided difference, especially on lens assemblies that
have a very convex front lens surface.

The bad effect on shapness, and bad color fringing, is due to the
optical defects that creep in, e.g. lateral and longitudinal chromatic
abberation, coma, etc.


An ordinary watertight camera can be made to work equally "sharp"
underwater as it does in air, by the simple expedient of adding a
watertight flat screw-on filter in front of the curved frontmost
element of the lens assembly. Now the lens proper is "working in
air", like the lens designers intended.

Assuming the camera, lens, etc. are all completely watertight,
everything should work just ducky, as far as sharpness is concerned.

Mark-
--
ACM, AAAI, EEEI

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