P3D Re: Blah, Blah, blah

["Dr. George A. Themelis" <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

Bob Maxey, I still think you don't get it.  The f8 marks in the DOF
scale can help you decide on the appropriate depth range.  Here is a
practical example:

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Say that you want to take a picture with the closest object at 4 feet.  
Trying to include 4 feet *and* infinity in the same picture will give 
you too much depth.  You want to restrict your furthest object.
But where?

Here is how to find the answer with a Realist with a hyperfocal scale:
Set the 4 feet (of the nearest object) next to the f8 mark of the scale.
Read the other f8 mark.  In the Realist that I have in front of me this
says around 7 feet.  This is where your furthest object should be.

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(Bob, you don't have to read the following... Yeah, will take it to 
tech-3d... as soon as the list shows some signs of life :-))

The reason this (seemingly unrelated tool) works, is that the DOF and 
deviation formulas are very similar.  This has been discussed a number
of times.

If that's not enough for your purposes, consult Tom's graphs.  Like Tom
has discovered, many of the rules of thumb break down in macro photography.
That's because the focal length of the lens changes to F(1+M) where M
is the magnification, plus some of the simplifications in the formulas
(like dropping the FL in front of the subject distance) do not apply
any more.  Quite possibly this rule with the f-stop marks breaks down 
too but the Realist (or equivalent stereo camera) is not really the proper 
tool for very close-up (1-2 feet) stereo work.

George Themelis


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