Alan asks:
<< would edge test differently if focus was optimized for the edge and
not the center ? >>
This assumes a camera is designed to focus in "the center." They aren't!
They incorporate "focus offset," which takes into account an average
curvature of field for the lenses in the manufacturer's lineup. When
you think you have focused in the center of your subject, you have
actually focused mid-way between the point of best center sharpness and
best edge sharpness for a typical lens, by design. So, neither center
nor edge ratings in film besed lens tests represent the highest value
possible.
<< has anyone taken into account curvature of field when rating edge
sharpness of a lens at close focus. >>
Not in any published literature. The only people who might be interested
in this are lens designers evaluating prototypes, or users who want to
rationalize that their lens with high curvature of field really can
produce a sharp off center image, albeit just not at the point the lens
is focused at. I ask: does one even know how far forward or back that
point is at every distance setting?
The 35mm SHIFT is a lens with strong curvature of field. In the last
test I did with this lens, I had to compromise between focusing on the
center and on the edge. It was the only lens I had to do this with.
Many Zuikos can't effectively be corner focused. Remember that a C+
grade or worse results in increasingly mushy viewfinder images in which
critical focus is more guess than technique, even when viewed through a
Varimagnifinder. In another words, you tend to hunt for best focus in
the corners, rather than have an image snap into focus, even with the
optimum focusing screen. Before anyone challenges me on that statement,
be sure to check my charts for the corner performance wide open of your
model lens. A 28mm f/2 user would challenge me, but that lens rates a B
in the corner wide open. It is a pleasure to use it on a tripod for
subjects requiring attention to depth-of-field, since you don't have to
reposition the camera to determine focus for any point in your field of
view.
Gary Reese
Las Vegas, NV
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