At 12:21 PM 7/5/04, Walt Wayman wrote:
>What am I missing? I hope we're not confusing coverage with field of
>view. To me, field of view is what you get on film, what you see through
>the viewfinder or on the ground glass, regardless of the image circle the
>lens may project, a.k.a. its coverage. And I do know about coverage. One
>of my favorite MF lenses is a super-sharp 90/5.6 Caltar. It will cover
>the 5x7 in. format, but I use it on my 6x9cm baby Graphics. Talk about
>cherry-picking the center out of an image!
>
>Walt, often Clueless, but eager to be instructed
I'll try to do this with ASCII-Art . . . best viewed with fixed-pitch font!!!
The light pouring through the lens is an image cone. The angle at its apex
is the acceptance angle, and the base of the cone forms the image
circle. Now make a cross-section of the cone on its axis which is a
triangle. Apex is acceptance angle and base is image circle diameter. In
a shift lens with much larger than needed image circle, the diagonal of the
film gate only takes out a slice of the triangle's base. What you observed
in doing your test is exactly what I would have expected:
--------------------
Figure 1:
/\
/**\
/ ** \
/ * * \
/ * * \
/ * * \
/ * * \
/ * * \
/ * * \
/_____|______|_____\
|^^^^^^|
Portion of image cone used with shift centered.
--------------------
Figure 2:
/\
/**\
/* * \
/* * \
/* * \
/* * \
/* * \
/* * \
/* * \
/|______|__________\
|^^^^^^|
Portion of image cone used when shifted left.
--------------------
I'm not surprised that you're seeing a shrinkage in the field when you
shift in any direction off-axis (center). I would expect that from the
geometry of the portion of the image cone you're using centered versus the
portion you're using shifted. The angle at the apex of the light cone for
the portion of the cone being used by the film gate shrinks slightly as it
is moved from center toward the edge of the cone.
-- John Lind
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