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From: gee@ihuxa.UUCP
Newsgroups: net.rec.photo
Subject: Nikonos question
Message-ID: <273@ihuxa.UUCP>
Date: Mon, 19-Sep-83 11:33:47 EDT
Article-I.D.: ihuxa.273
Posted: Mon Sep 19 11:33:47 1983
Date-Received: Tue, 20-Sep-83 22:41:38 EDT
Organization: BTL Naperville, Il.
Lines: 34

The lenses for the Nikonos camera are all marked in what should be 
considered apparent distances.  All one needs to do is set the focus at the
distance the object appears to be at.  Things do appear closer underwater
than they really are, but remember that if you see it closer, then the 
camera will see it as being closer (the camera lense woks the same way 
as your eye) (oops that is 'works' not 'woks').  The reason for the 
object appearing closer is that the light reflected off the object has to
travel through 3 mediums (water,glass,air) and due to the differences
of refraction there is magnification of the object, which in turn not 
only makes the object seem closer but much larger. As a guide the 
following may help:  an object that appears 4 ft away is actually 6 ft 
away -> a 2/3 change in distance,  also an object that is 4 ft long 
appears to be 5 ft long -> a 25% magnification.
As stated by someone else if you use a strobe you can be helped out
a little by the depth of field of the lense, but another problem to
think about is: if the object appears to be 4 feet away you focus at 4 
feet but you MUST aim the strobe at the actual position of the object.
Most strobes when used underwater do not cover a wide enough angle to 
illuminate the entire area being photographed (this is a greater 
problem with the 28 mm and the 15 mm lenses than with the 35 mm).  Also
a strobe is used mainly to add to the color spectrum rather than just
providing more light (the color red is absorbed by the water, so the 
red part of the spectrum is missing at depths greater than 10 feet)
this fact also makes strobes non-effective at camera to object 
distances greater than 6 feet (the light from the strobe travels 6 ft
to the object and then 6 ft back to the camera for a total of 12 ft -
in this 12 feet the red spectrum again is filtered out).
To get your best pictures you need to get as close to the object as 
possible.
The above measurements for change in distance and measurement are 
approximations (I do not remember the actual ratios, but its close),
I have found that most people have a hard time estimating distance 
underwater, just remember to focus at the distance the object appears to
be.