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
--
_________________________________________________________________
Options: http://lists.thomasclausen.net/mailman/listinfo/olympus
Archives: http://lists.thomasclausen.net/mailman/private/olympus/
Themed Olympus Photo Exhibition: http://www.tope.nl/
|