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[OM] So here's the deal...

Subject: [OM] So here's the deal...
From: usher99@xxxxxxx
Date: Fri, 17 Dec 2010 19:29:26 -0500
AG writes:
>The main reason why an internal-focus lens loses less light during
>close focusing is two-fold. One reason is the lens will shorten in
>focal length. The other reason is that depending on design, the
>focusing is done by optically moving the rear nodal point through the
>adjusting of lens elements in relationship to other lens elements,
>whereas a traditional lens physically moves the rear nodal point by
>physically moving the lens away from the film or sensor.

Well it seems many macro lenses and many zooms with close focus ability 
do shrink the focal length to gain mag.
Symmetric lenses at any given magnification won't gain any increase in 
effective aperture by changing focal length.
Seems to be an easy way to get increased mag w/o having a giant 
extension required for the helicoid mechanism.

Perhaps someone noticed the increase DOF of the longer FL lens in the 
toothwalker link with PMF of less than 1--


>From toothwalker link:
This is misleading!


VWDOF 2.1---------input---------------------
  Format            24x36 mm     24x36 mm
  COC               0.030 mm     0.030 mm
  Focal length      60 mm        100 mm
  Pupil factor      1.0          0.7
  F-number          8            8
  Magnification     1            1
  ------------------output--------------------
  Depth of field    0.960 mm     1.17 mm

Ha--this seems to violate the no free lunch rule of optics.. But NO!  
The effective aperture is governed by :
effective aperture= marked aperture  X (magnification/PMF  +1)
Thus the effective aperture # is 2.43 X marked aperture  for the longer 
FL lens while the shorter is only 2X the marked aperture or F16.
So when  adjusted for Effective Aperture , only  **Mag***  governs DOF:
http://www.zen20934.zen.co.uk/photography/dof/dof.htm

Thus there  is NO free lunch.

Why bother with this?  Emperically AG is correct regarding at least  
moving the rear nodal point, I think.  I have never
seen the math behind this or how that works.  In the wisdom of the OM 
archives, have seen others measure
light loss with macro lens with IF on a bronica etrsi with a sekonic 
meter in reflective mode where the film would be  demonstaring only 1 
2/3 stops lost at 1:1 rather then 2 stops.  This is about the same 
aswith the CV 125/2.5  Apo Lanthar using an OM meter.
Seems to be an optics sleight of hand --- at least intuitively 
violating the NFL rule of optics.

Mike
-- 
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