Re: [Leica] focusing (was zone f.)

[Henning Wulff <henningw@archiphoto.com>]

At 12:43 PM +0200 8/21/03, Christer Almqvist wrote:
>(resending this message)
>
>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.

You have it all correct, Christer. This is indeed a problem when you 
have to focus on something away from the center of the frame and then 
recompose.

The thing that makes this less of an issue is that depth of field 
does actually come to the rescue, as in the above example you would 
need at least a 21mm lens to get both children in, as a 24 would only 
include each of them to their centers. A 21 focussed at 5m still 
includes 4m within the depth of field even at f/2.8. The 4m object is 
not rendered as sharp, but you would rarely notice it.

With the 50/1 this is more of an issue, but at f/1 you usually have 
your main subject close to the center, if for no other reason than 
the rather severe vignetting.

I once read that this very issue meant that lenses with moderate 
field curvature (of the correct type) were better for rangefinder use 
than flat field lenses. The only problem with that is that then flat 
subjects are rendered unsharp at the edges.
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    *            Henning J. Wulff
   /|\      Wulff Photography & Design
  /###\   mailto:henningw@archiphoto.com
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