Re: [photo-3d] DOF & Rangefinder

[Brian Reynolds <reynolds@xxxxxxxxx>]

George A. Themelis wrote:
> Chuck writes:
> 
> > I really don't know what is meant by "I don't use a rangefinder, I
> > use maximum DOF."  All I can assume is that they use their
> > tightest F stop and focus for about 15 feet.  The problems with
> > this are the tightest F stop rarely produces the sharpest photos
> > and many stereo cameras vignette badly at F22.
> 
> Chuck, do you know what the hyperfocal distance is?  I don't think
> you do.  So let me tell you:
> 

I'm fairly sure Chuck does know what hyperfocal distance is.  He just
has different objectives than many others and uses a slightly
different way of describing it.

> For every f-stop there is a distance associated with it, called the
> hyperfocal distance.  This gives the maximum DOF for this given
> f-stop.  If you focus at the hyperfocal distance for the given
> f-stop then your Depth of Field (DOF) extends from half that
> distance to infinity.  People who focus at the hyperfocal distance
> do not necessarily use the "tightest" i.e. smallest f-stop.  But
> they want to have infinity in focus and as much in the foreground in
> focus as possible.
> 

That last sentence is not correct.  If you focus on the hyperfocal
distance for the aperture that you are using, infinity and an
arbitrary point (half the hyperfocal distance) will be equally
(un)sharp.  They will not be in focus.  Only the distance that you
focused at (the hyperfocal distance) will be in focus.  If you want a
particular distance (infinity, the near point or some where in
between) in focus you must focus on that distance.

It's good to remember that the hyperfocal distance for a particular
aperture is an arbitrary distance.  It depends on the size of the
circle of confusion on the film.  The circle of confusion is the
largest blurry spot that is still considered "acceptably" sharp.
"Acceptably" sharp is a subjective measure.

Due to its poor focusing screen I used hyperfocal distances to
maximize DOF with my Sputnik.  I found that more often than I liked a
close portion of the main subject was too soft while some unimportant
point further back was sharp.  I now make the effort to focus on the
most important part of my subject and then choose the aperture to give
me the desired DOF.  Of course I'm a heretic and believe that
selective focus is just as useful in stereo photography as it is in
regular photography.

By the way, it is possible to have a near point, mid point and
infinity all in focus at the same time, but to do so you need a camera
that has lens swing and/or tilt.  Keeping the lens axis perpendicular
to the film plane means that only one distance from the camera (as
measured from the film plane) can be in focus.

-- 
Brian Reynolds                  | "Dee Dee!  Don't touch that button!"
reynolds@xxxxxxxxx              | "Oooh!"
http://www.panix.com/~reynolds  |    -- Dexter and Dee Dee
NAR# 54438                      |       "Dexter's Laboratory"