I really have no idea but would suggest that it's possible the Big Foot
reduces its focal length when focusing at macro ranges such that the
f/2.5 aperture at 125mm might be, say, f/2 at 100mm.
Just sayin'
Chuck Norcutt
usher99@xxxxxxx wrote:
> 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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