DEPTH of FIELD

[P3D Paul S. Boyer <boyer@xxxxxxxxxxxxx>]

John Roll has asked a question about depth of field.

DEPTH of FIELD Calculations

The first step is to find the hyperfocal distance for your setup.  If
one focuses at the
hyperfocal distance = H, everything should be sharp from 1/2 H all
the way out to infinity.

I would use a circle of confusion of 0.03 mm.  The H is then

H = (F^2)/f*d

where F^2 = the focal length of your lens, squared; f = f-stop
number;
and d is the diameter of the circle of confusion.  Use all metric
units, all meters,
except that if you set it up this way

Hm = (Fmm^2)/f*30

you can get your answer for H in meters, and put into the formula
your
lens-focal length in mm.  In your case of a 30-mm lens, this works
out nicely, that

Hm = 30/f

So for an f-stop of 16, Hm = 1.875 m
With a speadsheet program such as Excel, you can make yourself a
little
table of hyperfocal distances -- or tables for all lenses in the
house --
or interval tables for zoom lenses.  Play with changing the d-value:
there's
no end to the fun of it, if the computer does the calculations for
you.

Now back to your problem.  The next step is to calculate the near
limit and
the far limit of your depth of field.  The formula for the limits
(these work until you get
way down into the macro range is

Near limit = (H*u)/(H+(u-F))

Far limit = (H*u)/(H-(u-F))

where u is the distance for which the camera is focused.  For your
case where
u = (approx) 390 mm

Near limit = 1.875*0.390/(1.875+(0.390-0.030) = 0.327 m

Far limit = 0.73125/(1.875-0.360) = 0.483 m

A good source for this sort of calculation is the Kodak-Amphoto
"Encyclopedia of Practical Photography" (1978).  It is just old
enough that it includes
all sorts of good calculations which people do not use with modern
automated cameras.  It has excellent articles, including a fine on on
stereo photography.  The articles are unattributed, but I once did a
little research
and learned that the latter article was written by W. A. Young, who
worked for Kodak, and then retired to Arizona.  That article is one
of
the best summaries on stereo for a starter that I have seen.
The rest of the Encyclopedia is also filled with great information.
There was a later, similar Encyclopedia which was an artsy treatment
of photography, but it was nowhere near as useful to anyone
interested in
the technology of good results.  In photography, technology comes
first,
then artsiness.

Modern text-books on optics are similarly discouraging to one who is
looking
for practical solutions.  That is why I refer to Hardy, A.C., & F. H.
Perrin,
1932.  The principles of optics.  McGraw-Hill Book Co., New York,
xiii, 632 pp.
It even has a chapter on stereoscopy: Ch. 25, pp. 517-533.  (I would
love
to find a copy, but have had no luck so far.  I only know of one in a
library,
whose librarian has been threatened with severe reprisals if he
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should
discard this book in the mistaken impression that it is
"out-of-date.")

Best of luck with your experiments.

--Paul S. Boyer   <boyer@xxxxxxxxxxxxx>


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