Archived posting to the Leica Users Group, 2003/01/14
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let me try.
i think this is the idea. object "O" in center of plane of focus for
lens "L", distance OL is 100cm.
-------O-------
\ | /
\ | /
\ | /
L
now rotate L counterclockwise (or O clockwise around L)
distance OL is still 100cm. now create a plane intersecting
point O perpendicular to the lens axis, and define point X as the
intersection of that plane and the lens axis. distance XL is
less than OL. if view angle of lens were 90 degrees, than the
distance XL would be about 71 cm (100/sqrt(2)).
if we assume that lens L renders a sharp plane of focus
perpendicular to its axis, then X and O are on the same
plane of focus. therefore to get O at the edge of the frame
in focus, you need to set focus to 71 cm, despite the fact
that the object is 100cm away from the lens.
---------------
--\-----X----O--
\ | /
\ | /
L
forgive the sloppiness, i have not done trig or geometry
in the latter half of my life, and i was never a particularly
able practitioner. :-)
- -rei
On Tue, Jan 14, 2003 at 10:14:30AM -0800, Herb Kanner wrote:
>
> I absolutely don't understand Martin Tai's geometry. He claims that
> if the subjects eye is 100 cm from the camera as indicated by the
> scale on the lens barrel and is in the center of the image, that if
> the camera is rotated so that the eye is now at the edge of the
> frame, it's distance will have been reduced to 94.7 cm. This looks
> like utter nonsense. Can anyone prove it to be correct?
>
> Herb
>
> -------------------------------------------
> Original Message:
>
> For those who haven't seen it, I can recommend Martin Tai's Leica M & Rs
> recompose focus guide. Great for understanding how to be able to compose
> shots that are not head on while maintaining precise focus even at full
> aparture, this was a bit hit and miss for me before even with SLRs
> (although easier).
>
> http://www.accessv.com/~martntai/public_html/Leicafile/recompose.html
- --
Rei Shinozuka shino@panix.com
Ridgewood, New Jersey
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