Archived posting to the Leica Users Group, 1998/04/20

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Subject: Re: [Leica] M's and focusing
From: "Art Searle" <w2nra@erols.com>
Date: Mon, 20 Apr 1998 22:07:50 -0400

Yods1 wrote:
>Technically, it seems to
>me that if the field-plane is truly flat, that will back-focus the eyes of
the
>person when I change the composition.

I think this is a good question and raises a very valid point.  I don't
know the answer as far as optical theory goes but I did do the math
(trigonometry).  If you take a 35mm lens focused on a subject 10 feet away
and then recompose so that your subject is at the very edge of the frame
that subject is still 10 feet away and the lens is focused to 10 feet.
However, if the lens has a flat field it would be in focus at 10 feet only
in the center and at 11 1/4 feet at the two edges of the frame, so the
subject could be out of focus.

If this 35mm lens were set at f/1.4 the depth of field in front of the lens
is only 1 foot and your subject would be out of focus at the edge of the
frame if the lens had perfect flatness of field.

I never thought of this Yods1, the math proves your point.  Does optical
theory?  Is the flatness of field truly that flat?

Art

Art Searle, W2NRA, w2nra@erols.com, Lake Grove, Long Island, NY, USA
20 miles east of Nikon USA, 70 miles east of Leica USA