Archived posting to the Leica Users Group, 2003/01/14

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Subject: RE: [Leica] I eat crow
From: "Austin Franklin" <darkroom@ix.netcom.com>
Date: Tue, 14 Jan 2003 22:51:30 -0500

Henning,

> Really, Austin. You have to _read_ the last message in which I
> explained the geometry of focussing.

The page that we were discussing, and that I claim is wrong, does not
discuss what you are talking about as it is currently worded.  Your premise
is that you are "re-composing" by turning the camera so the subject is now
off to the side of the finder.  For that premise, you are correct, the focus
distance changes, as the plane to plane distance changes.  BUT...if you
merely step to the side to re-compose, as it is clearly stated on that web
page:

"shift the camera side way"

Which means basically to step to the side, planar with the film plane, so
the model's eye is now off to the side of the finder.  Shift does not mean
turn.  Perhaps the person who wrote the page meant turn, as it would
seem...but that is not what it says.  I do know him, and will offer a
correction.  Your plane to plane distance has NOT changed, therefore there
is no need to modify your focus.  A technique that can be used to avoid
having to do any "calculations".

I agree my original response was not well written, and could be misconstrued
as to what I was trying to say.  I'm sorry for that.

> Focussing is not on an arc, it's on a plane. The rest follows.

Read my 2nd to last post in response to Johnny, and you will see what I was
talking about, and it is different than what you are talking about.  Finding
a focusing aid must be done along an arc if you are standing in one place
and turning the camera to find that focusing aid.  If you don't focus on a
point on the arc that is the same as your subject distance, you will not
focus at the correct distance.

Austin

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