[photo-3d] Re: depth of field

["Dr. George A. Themelis" <drt-3d@xxxxxxx>]

DOF tables are calculated, not measured.  Three factors
enter in the calculations:

1) Focal length
2) Aperture size (f-stop)
3) Circle of confusion

Since being out of focus is something gradual and only one
plane at a time is in focus, factor no. 3 is an arbitrary 
constant which determines how much "out of focusiness" is
acceptable.

Lens manufacturers have more or less agreed in given
circle of confusion values to be used as a function of
the film format.

Based on these facts, I would expect that all 35mm lenses
will list the same DOF at the same aperture if the same
circle of confusion is used in the calculations.

It is known (and has been discussed in photo-3d) that
David White used a smaller (more strict) circle of confusion 
in earlier calculations so earlier Realists show LESS
DOF than later Realist.  Realist changed this value and
their DOF tables later on and explained this change in
their newsletter.  This is only a calculation issue.
The lenses will not show different DOF. 

You mention that for F8 the Realist and the FED BOY give
different DOF, from infinity to 9 feet and to 10 feet.
Is this something measured or taken out of tables?

Where is the lens focused?  At infinity?  (You cannot
quote a DOF range without indicating where the lens is
focused unless if you are talking about the Hyperfocal
Distance. If infinity is the outer range of the DOF
then most probably you are talking about the Hyperfocal
Distance, which is the twice the shorter DOF limit and
that's where the lens if focused.)

George Themelis


>Can anyone tell me what factors govern a 35mm lens' depth of field?  Does a
>greater DOF indicate a better quality lens, or one of a greater diameter, or
>what?  I was trying to predict what my DOF would be with my Wirgin stereo,
>so I looked at my other cameras to see if they all run about the same.  They
>are similar, but far from identical in this regard.  I see that my Realist
>3.5 at (for example) F8 will give good DOF from infinity to 9 feet; by
>comparison my FED B-O-Y at F8 is sharp from infinity to 10 feet; and my
>Realist 45 DOF at F8 extends from infinity to about 6.5 feet.  All of these
>lenses are supposed to be 35mm.  So I'm wondering, does the greater DOF of
>the 45's Steinheil Munchen Cassars indicate better lens sharpness overall
>than the David Whites on my Stereo Realist?  If not, what is the reason for
>this DOF variation?