I agree with AG AND Dr. Focus. Focusing accuracy is a key parameter
and need to meticulously exclude
focusing differences to explain any findings.
I was home yesterday and tried another focus stack (perhaps a futile
attempt) (with gross calc dof in hand)
There was a pesky breeze and the light was changing and getting 8 good
images was not easy (probably actually not accomplished) . I think
with mag (10X) LV
one can discern some differences within the calculated dof.
Bracketing the mid point of the proximal and distal dof limits as
well as multiple repetitions should
minimize the effect of this variable.
Dawid is of course correct for an ideal lens, though with any complex
lens subtle changes in the dof would be expected.
I have trouble differentiating the magnitude from the "rendering." of
the defocused area though I understand what he is getting at. If it
looks sharper it may as well be called sharper as it is a visual
endpoint.
Perception is reality in this realm for all practical purposes , IMO.
Then one must also be careful where in the frame the focus point is
because of
possible field curvature. There also may be unknown unknowns, but I
don't know.
Even thinking about complex zooms with asymmetrical elements makes my
head hurt.
Undoubtedly Zeiss has full wave solutions for most their lenses and
could just calculate this stuff despite the somewhat arbitrary nature
of determining the Coc.
That some of these effects are not trivial is born out by the Zeiss
focus shift link.
If I like this thread does that mean there is a problem, Mike
I'm going to AGAIN do the tape-measure test. Not to prove anything to
you, but to answer this lurking question in my own mind as to why the
two lenses aren't focusing the same even though they appear to be
focusing at the same point. The Zeiss article definitely points to
this as being either a nodal-point difference or a focus shift due to
aperture adjustment. The Zeiss article also addresses the cone
differences at identical apertures from lenses of different maximum
apertures.
In the world of assymetrical lens elements, all bets are off.
AG
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