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Re: [OM] Macro light loss-continued

Subject: Re: [OM] Macro light loss-continued
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Wed, 05 May 2010 22:33:51 -0400
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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