T3D Re: Underwater stereo photography

[john bercovitz <bercov@xxxxxxxxxxx>]

Michael Kersenbrock writes:

> F.L. in Air   F.L. in Water/Air
> -----------    ----------------
>     21               28
>     28               37.3
>     35               46.7
>     50               66.7

Sure looks like the focal lengths are related by the 
ratio of the index of refraction of water to the index
of refraction of air which is quite close to 4/3 at room 
temperature.

Bill Costa quotes http://www.cyber-sea.com/htmfiles/art3.htm

 "Divers learn in their first open-water training classes that
  objects underwater look 25 percent closer and 33 percent
  larger when viewed through a standard mask, equipped with
  'flat optics'."

I think they are saying that if something is 25% closer it is 
3/4 as far away and so subtends an angle which is 4/3 bigger.

John B


------------------------------