Archived posting to the Leica Users Group, 1999/08/31

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Subject: RE: [Leica] Depth of field... (more than you wanted to know)
From: Paul Chefurka <Paul_Chefurka@pmc-sierra.com>
Date: Tue, 31 Aug 1999 05:35:53 -0700
 
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