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Re: [OM] Fwd: Paphiopedilum Orchid focus stack

Subject: Re: [OM] Fwd: Paphiopedilum Orchid focus stack
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Sun, 21 Dec 2014 21:12:54 -0500
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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