Re: [Leica] DOF? inaccurate adages

[Mark Rabiner <mrabiner@concentric.net>]

Mikiro Mori wrote:
> 
> Hi, I have a beginner's question.  What is a definition of DOF?  I assume
> that a plane of focus has no depth or thickness and that resolution/contrast
> is a function of the distance from the plane....   Such functions should
> depend on lens design and may explain lens characters....  If so, DOFs
> calculated with a single formula have limited significance in real shooting.
> Could someone please enlighten me.
> 
> Cheers,
> 
> Mikiro

This just got be thinking there was an article a few years back in a
better photomag about how a dozen of photography's basic adages were
said to be counterfactual. 
The First one I thought of here was the one third rule on depth of
field. When shooting fast you go one third back from the front plane of
focus with the idea being there would be 2 thirds after that point
you've focused on behind it that would be in focus. A basic rule that
most of us know and use and don't question but this guu took a handful
of lenses from different formats and tested them and some were not even
close and applying that rule to those lenses would not be smart.
If that were the case how could a depth of field chart working on
mathematical principles be really accurate?
I think this might in effect be Mikiro's question.
Also what were some of the other ones he had come up with does anyone
remember. One of his examples were later disproved or reproven a month
or two later.
The only other one I can think of is "expose for the shadows and develop
for the highlights" which as a working practice is almost never the
case. 
I'll think of another on undoubtedly at 1 in the AM.
Mark Rabiner