Archived posting to the Leica Users Group, 2003/08/20

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Subject: [Leica] focusing (was zone f.)
From: Christer Almqvist <chris@almqvist.net>
Date: Wed, 20 Aug 2003 21:48:26 +0200

OK, I  know the answer to this question could be: 'go and do some 
test shots', and also that the question may be slightly on the 
theoretical side, but anyway, here we go:

Imagine for a second that you want to make a picture of three people 
standing on the other side of the street. The street is 4 meters 
wide. One of the people, let us call him 'the father' stands directly 
opposite you on the curb, four meters away from you because you also 
stand on the curb. Also on the curb on the other side of the street 
from you, but three meters to the left of the father is his son, and 
similarly, three meters to his right is his daughter. Obviously the 
children are five meters away from you.

In this situation most people using an M will focus on the father 
because he is so conveniently located  right there where the little 
yellow window is, that we use for focusing. Others will focus on the 
father because they have heard something about flat film plane and 
they take that to mean that everything at a 'picture plane' paralell 
to the film plane will be at the maximum sharpness if the distance 
set equals the the minimum distance from the film plan to the picture 
plane (i.e. measuring at 90° from camera to picture plane).

Now imagine you want to take a picture  of just the children, but 
with the children remaining in their original position. In this 
situation, most people would focus on one of the children (and thus 
set the distance to 5 meters) and then reframe the picture to include 
both of them. With the flat film theory as I understand it, the 
children would not appear as sharp on the negative as they would had 
you not refocused, i.e.  they would have appeared sharper on the 
negative had you not changed the distance setting from 4 to 5 meters.

Please tell me what is right and wrong in the above. Please do not 
tell me that it does not matter because it will be covered by depth 
of field, because I know that is not true, at least not for large 
lens openings.

Rgds


- -- 
Christer Almqvist
D 20255 Hamburg and / or
F 50590 Regnéville sur Mer

please look at my NEW  b+w pictures at:    http://www.almqvist.net/chris/dozen/

old pictures still at:     http://www.almqvist.net/chris/new
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