Re: [OM] Zooms at Dawn - the 50mm shootout - Teaser

Subject: Re: [OM] Zooms at Dawn - the 50mm shootout - Teaser
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Wed, 21 Apr 2010 09:23:05 -0400
Well, the inverse square law isn't broken but in John Colishaw's 
"Closeups in Nature" he gives examples using a 50mm lens, a 2X 
teleconverter and various extension tubes applied alone or ahead of or 
behind the converter that all result in a 1:1 image but have differing 
working distances and differing degrees of light loss... ranging through 
2, 3, 3-1/2 and 4 stops.  Sounds a bit strange at first until you 
realize that the effective focal length is also changing which is why 
the working distance is also changing.  Least light loss (2 stops) is by 
simply adding 50mm of extension to the 50mm lens to get 1:1 and a 4" 
(100mm) working distance.

Google has some of Shaw's book on line which can be gotten to by this 
link.  Click the highlighted text (which doesn't really take you to the 
right spot) and scroll down to and read pages 110-111 for details. 

The Vivitar MFTC varies its length by varying the extension between the 
lens and TC lens.  When the MFTC is at 1:1 this is equivalent to Shaw's 
third method of reaching 1:1 by adding 25mm of extension between the 
50mm lens and the 2X converter.  This method increases working distance 
to 6" but also costs 3 stops instead of 2. (1 to the extension, 2 to the 
converter).  But it may be better than Shaw's second method which is to 
add 100mm of extension behind the lens mounted directly to the 
converter.  That yields a 100mm lens and 8" working distance but also 
costs 4 stops and quite a long lens/tube configuration.

I think I'm gonna shoot inside today.  I've had enough of the wind.  :-)

Chuck Norcutt

usher99@xxxxxxx wrote:
>> usher99@xxxxxxx wrote:
>>  Must measure with Big Foot at 1:1 as it uses a combo of
>> standard
>> extension and floating elements--down side of straight extension is
>> loss of light of course.
>> Mike
>>From my understanding the light loss is directly related to 
> magnification, so
>> whether that 1:1 is achived by changing focal length, lens extension 
> or a bit
>> of each is irrelevant.
> ...Wayne
> Wayne Harridge
> Well it is a good point that the no free lunch principle applies--more 
> mag means more light loss and a smaller fraction of the total image 
> circle is used.
> The inverse square law should apply.
> The exposure correction factor should be able to be calculated from 
> (M/P+1)^2 where M is the magnification and P is the lens pupillary 
> magnification --I think about 1 for a standard lens).
> For IF lenses things are complicated. . These lenses focus by moving 
> the rear node away from the film/sensor this results in some shortening 
> of the focal length and a change in pupillary magnification results.  
> This should lead to slightly less light loss than a linearly focusing 
> lens. I believe this to be true and have read it from several sources.  
>   It seems it should be possible to compensate
> in front to keep the FL/working distance the same, but I have no clue 
> whether such a lens has been made like that.
> I can tell you the VF is much brighter at 1:1 with Big Foot than the 
> viv S1 90mm  F2.5 with matched TC at 1:1.
> I would defer to Dr. Focus or Dr. No-Flash on such matters as either 
> have forgotten more about such things than I have learned.
> Mike
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