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Re: [OM] pics of shift lenses

Subject: Re: [OM] pics of shift lenses
From: "John A. Lind" <jlind@xxxxxxxxxxx>
Date: Fri, 04 Feb 2000 18:39:52 +0000
At 05:32 2/4/00 , you wrote:
>When the term "shift" or "perspective control" are used, I always conjured
>up images of a mini-view-camera; or in the case of OM, a lens in flexible
>bellows that allowed the tilt/shift. However, looking at the eSIF, much to
>my surprise, the 35 and 24 Zuiko shifts are very small, compact, and most
>of all elegant. Where can I see images of them tilted/shifted, and
>how/where do they bend from? Also, since each respective lens has an image
>circle (correct term?) of a lens of shorter focal length, where does that
>excess go?

I think the sliding part has already been touched on.  The portion that
slides around is between the rear mount that mates to the body and the rest
of the lens.  That is why they are manual stop-down.  There's no simple way
to link the auto stop-down to the body.

All you are doing with the shift is moving the larger image circle around
the film gate and picking what part of it will end up on the film.

Most better non-shift lenses also have an image circle larger than the film
gate.  It's not quite as oversize as in a shift.  The reason is to reduce
the cos^4 vignetting (a.k.a. falloff) at the edges.  This affects all
lenses.  Edge (off-axis) ray paths see an aperture not quite as big as
central ray paths.  In a rectilinear lens, an object near the edge will
also be spread across a larger area of the film than the same object near
the center.  Both of these reduce its brightness.  By making the image
circle a little larger than needed, the most severe part of this effect
falls outside of the film gate.  There are some other optical tricks
performed inside the lens that allow the aperture to virtually "tip" some
(versus physically doing so) so that it has nearly the same diameter
center, or edge.

-- John

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