Ha,
Had to comment when I saw this description of Big Foot. Don't get me
wring it is a VERY nice lens but this one in Nykon mount
is an extra 1K for being able to violate the no free lunch principle,
AKA the 1st Law of thermodynamics: Moose is right about the web being
an effective way to initiate and maintain fallacies. (Other
mistatements there too--perhaps the fastest at that FL that goes 1:1)
http://tinyurl.com/29eahjp
Excerpt: caps are mine for emphasis
"Product Details: The Voigtlander 125mm f 2.5 APO-Lanthar has 11
elements in 9 groups. With 9 aperture blades, making it round for very
smooth and pleasant bokeh. The lens has a brightest aperture of 2.5
making it the fastest macro lens in the world. Not only that, most
macro lenses when focused to 1:1, some light will be lost due to the
high magnification. The Voigtlander 125SL is an exception. The lens
keeps it's brightest aperture at 2.5 EVEN AT1:1. Many might not need
the 2.5 aperture in macro, but f2.5 helps to keep the viewfinder bright
at all times, thus easier in manual. If one needed the slowest speed,
you can stop the lens down to the smallest aperture of f22."
Measured light loss using the technique from JS--evenly lit wall,
cloudy day---Light lost from infinity to 1:1 by light meter on OM-2
about 1.5
stops. That still seems weird (should be about 2) but less weird
than zero.
Related old conundrums related to that technique to measure light lost
from extension or TC.
Viv 90mm S1 matched TC --- 2 stops lost---PERFECT
Used Z. 50/3.5 macro in front of these:. Tamron SP 7 element 2X TC is
a bit
over 1 stop.
Tokina 2X TC 4element is a 1/3 stop less . OK, the slightly less
light loss is from fewer interfaces but the absolute # stops losts seems
Impossible--double the FL with same size physical aperture--must be 2
stops. arrgh
Perhaps there is something to JS statement that the macro lenses with
some floating elements with IF loose less light, but still
don't see how.
One of the list's best physicists has been too busy being productive
capturing nice images
to ponder such issues.
Now that all is clear, must get back to work.
Mike
--
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