Archived posting to the Leica Users Group, 1999/07/08

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Subject: Re: [Leica] DOF?
From: "R. Saylor" <rlsaylor@ix.netcom.com>
Date: Thu, 8 Jul 1999 18:22:26 -0400

Any distance other than the plane of focus is going to be out of 
focus. The near and far ranges of the DOF are supposed to be the 
minimum and maximum distances for which objects will not be so out 
of focus that it will be obvious on a 8 x 10 print viewed at a certain 
distance (I forget how far that's supposed to be). The DOF formulas 
depend on the size of the circle of confusion, which is a measure of 
how out of focus something is. It is purely optical theory and doesn't 
take into account lens characteristics.

Theoretically, a perfect mathematical point which is out of focus will 
produce an image which is a disc, or circle, rather than a point. The 
diameter of this disc is the circle of confusion (COC). The largest 
allowable COC determines what is known as "acceptable" focus. The 
smaller the allowable circle of confusion, the sharper the acceptable 
focus. Zeiss seems to favor 0.025mm, while Leica seems to use 
0.033mm. Using a smaller COC in the DOF formulas results in a 
shallower DOF. 

The COC is merely a way of making the idea of "acceptable" focus 
mathematically precise. What COC to use in the formulas depends 
on the size of the enlargements you wish to make, the viewing 
distance, the prominence of the film grain, etc.There are so many 
variables that it's hardly worth bothering with.

Richard S.

Date sent:      	Thu, 8 Jul 1999 12:10:53 +0200
To:             	leica-users@mejac.palo-alto.ca.us
From:           	Mikiro Mori <arbos@silva.net>
Subject:        	[Leica] DOF?
Send reply to:  	leica-users@mejac.palo-alto.ca.us

> 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
> 
> 
>