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

["Jack Herron" <jherron@theriver.com>]

Chris,
    The confusion lies in the "flat field correction" concept.  The only
lenses that are corrected for both sides (object and film plane) are
so-called process lenses, used to photograph flat material onto flat film,
such as "burning" an offset plate from a photo.  In this case both film and
object are flat, and this kind of correction is required.  In an uncorrected
lens (for this aberration) both planes of focus, in front and back, are
spherical.  In a camera lens, the film plane is flat, and so the lens is
corrected to focus on a flat plane on the film side.  No attempt is made to
correct the plane of focus in front of the lens, as most real objects we
will be photographing are not planar.
    Hence, to get the children in focus, focus on them, and re-frame.  Dad
will be out of focus by a yard, which will likely be covered by DOF.
Cheers,
Jack

Jack Herron
8118 E. 20th St.
Tucson, AZ 85710
520 885-6933
- ----- Original Message ----- 
From: "Christer Almqvist" <chris@almqvist.net>
To: <leica-users@mejac.palo-alto.ca.us>
Sent: Thursday, August 21, 2003 3:43 AM
Subject: [Leica] focusing (was zone f.)


> (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.
>
> 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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>


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