Thanks Dr. Focus. I knew you would have an answer.
Perhaps this also explains how in an extreme stacking case going from
1:1 to infinity it seems optimal to keep the entrance pupil stationary
and move the cam on a bellows back to get more mag. The lens must
perform well at higher mag which may not be the case for many wide
angles to produce these "macro landscapes." The Zuiko 28 seems to
work quite well by report though the only rig I have seen using it
required mount surgery and putting it on a Minolta bellows. I am not
sure how else to accomplish this feat. Perhaps a T2 mount or Mamiya
Wide angle could be mounted on an OM bellows as long as the
registration distances and minimum extension on the bellows allow for
this. I think there is a cheap T2 mount vivitar 28mm that might work
but no clue as how it would perform.
Contemplating an extreme stack, Mike
Dr. Focus writes:
If you will refer to the diagram and formulas on this page
<http://www.mhohner.de/formulas.php>
Note on the diagram that the value g is the distance from the object
(that's being photographed) to the optical center of the lens. The
value b is the distance from the optical center of the lens to the
image
plane. Next note that magnification is given by g/b
If, when taking images for stacking, you leave the camera stationary
and
refocus by moving just the focusing ring forward you are moving the
optical center of the lens away from the image plane and increasing the
value of distance b. Distance g is presumably staying the same. If
you
move the focusing ring forward by 2mm the distance g can stay the same
by virtue of moving the focus point forward by 2mm as well. But since
the camera and focal plane didn't move the distance b is increased by
2mm. Since magnification is given by g/b and b is now larger the
magnification will be decreased slightly.
On the other hand, if you move the camera and lens together on a slide
as a unit the distances g and b never change. The focus point is
moving
along with the motion of the camera and lens but, since neither
distance
g nor b changed the magnification didn't change.
Now for the kicker. Each time you move the slide you are also
rephotographing an area that you already photographed, But, since *it*
is now at a distance different from the prior photos its (less focused)
part of the image is having its magnification changed. Hopefully, this
will be the part of the image that the focus stacker decides to throw
away. In a nuthshell... go with the slide mechanism.
Dr. Focus
On 12/21/2014 3:42 PM, Mike Gordon via olympus wrote:
The framing seems to change less with using a rail--you'd have to ask
Dr. Focus as to why. (yes, I have checked both methods with the same
shot)
--
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