[Leica] RE: Depth of field... (more than you wanted to know)

[Jim Brick <jimbrick@photoaccess.com>]

Paul,

What I recited is not only what I learned in school, 40 years ago, it is
exactly the same thing that is written up in ALL good photographic theory
texts. And you have to remember that GIB did not ask about "hyperfocal"
focusing. Hyperfocal focusing still follows the optical law... acceptable
blur (Circle of Confusion). So the hyperfocal point, and associated "in
focus" area, will change depending upon whether you want acceptable images
or "dead sharp" images. The hyperfocal point is special "only" because it
references infinity. But in photographic terms, a lens manufacturer
specifies infinity as a "few hundred meters." Not really infinity. So all
of the DOF laws are in tact. Even when using the hyperfocal point. Which
will probably be different for me, than for you.

Jim

The word "infinity", in photographic terms, is misleading.


At 05:35 AM 8/31/99 -0700, you wrote:
>I agree with everthing in the post except for this oft-repeated "rule":
>
>> From the exact point of focus, DOF extends 1/3 forward
>> (toward the camera) and 2/3 back (away from the camera).
>
>The trivial refutation of this is the hyperfocal distance.  At this focus
>point, the DOF will extend some finite distance in front of the point of
>focus, but infinitely far behind it.  This is certainly not a 1/2 ratio.
>
>I'm looking at my 35/2 Summicron DOF scale right now.  Here are some data
>points using the f/16 marks:
>
>1.0m - near: .73m, far: 1.6m. Ratio = 1/2.2
>1.5m - near: .94m, far: 4m.   Ratio = 1/2.6
>2.0m - near: 1.1m, far: 10m.  Ratio = 1/8.9
>2.7m - near: 1.3m, far: infinity.  Ratio = 0
>
>So, my question is:  What is the scientific justification for this "rule of
>thirds"?  Empirically it doesn't seem to exist.
>
>Paul Chefurka
>