P3D Re: To Focus or not to Focus?

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

>I don't understand. Why isn't it a good idea to focus on your subject 
>all of the time? I do this all the time in both 2-D and 3-D photography. 
>Am I doing something wrong by making sure my subject is in focus? I 
>certainly don't want an out of focus subject!

I am sure you will get a few responses to this, so I rush to be the
first! I can see Bob Aldridge's fingers typing already! :-)

Yes, I want my subject in focus.  But where is my subject???

When I take a picture of my family at around 10 feet and the mountain
Rainier in the background and trees and nice scenery in-between, where
exactly is my subject?  Most probably I am more interested in my family
in this nice setting, so my subject is at 10 feet.  But if I focus at 10 
feet I will have Mt. Rainier out of focus.  So, I focus at 15 ft and I 
have both my family and the mountain in focus.  How is this possible?

Introducing the concepts of depth of field (DOF) and hyperfocal distance...

(I am sure many of you are familiar with these concepts so I writing 
here for those who might not be...)

When we focus at a certain distance, there is a certain area of "acceptable
focus" both in front and behind this distance.  This is known as the
"depth of field".  For example, according to the Realist manual, with
an f8 aperture and focused at 8 feet the DOF is from 6 to 29 feet.
The DOF depends in the aperture and the FL of the lens used.  Small
apertures (f11, f16, etc.) and short FL lenses have wider DOFs.

"Acceptable focus"?  Smells trouble...  Yes, indeed!  The derivations
of the DOF formulas use a constant called the "circle of confusion",
the smallest detail that can be resolved by the film or lens.  So in 
reality the concept of DOF is valid only if you accept the size of this 
circle of confusion upon which the numbers are derived.  

Hyperfocal distance:  If you move your focusing from close to further
away, there will be a point when the DOF will extend to infinity.
This is the hyperfocal distance.  For example with f8 in a Realist,
the hyperfocal distance is 15 feet.  If you focus at 15 feet with
your Realist at f8 then everything from half this distance (7.5 feet)
to infinity, will be in focus.  Focusing in the hyperfocal distance
gives you the widest possible depth of field.

For general stereo photography with infinity included in the picture
it is recommended that you focus at the hyperfocal distance and that
you don't change this, no matter where your main subject is (assuming
that infinity is always included in the picture).

Here is a Table of Hyperfocal Distances taken from Realist instructions:

Aperture  Focus at     DOF is from-to
--------  --------  ------------------
  2.8       44 ft    22 ft to infinity
  3.5       34       17    to infinity
  4         30       15    to infinity
  5.6       22       11    to infinity
  8         25       12.5  to infinity
 11         11        5.5  to infinity
 16          8        4    to infinity
 22        5.5        2.8  to infinity

Will conclude with a few warnings against the blind use of hyperfocal
focusing.

George Themelis


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