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[OM] Re: adding to macro

Subject: [OM] Re: adding to macro
From: Moose <olymoose@xxxxxxxxxxxxxxx>
Date: Sat, 16 Jul 2005 19:12:06 -0700
Wayne Culberson wrote:

>I think a 25mm extension tube will take the 50/3.5 to 1:1. I'm not sure how
>much extension is needed for the 90/2, but I would guess about 50mm of
>extension. 
>
The optical rule is that extention to the focal length is requires for 
1:1 and to 1/2 the fl for 1:2, so the 50/3.5 requires 25mm and the 90/2 
requires 45mm.

>A 25mm extension tube can be made from a basic 2x converter by
>removing the glass. They are practically free. 
>
The very common and cheap used Vivitar 2x-21 telexender can be very 
easily converted into a 26mm auto tube. Just remove the 3 screws on the 
back and carefully work the rear assembly off. Unscrew the rings and 
such holding the lens elements, remove the elements, remount the rear 
assembly, and you are done. Two of theses stacked would work for the 
90/2, although you should check the auto diaphram action to make sure it 
works correctly through two relays.

>Extension tubes produce light loss only.
>  
>
True, but no more than if the focusing helicoid extended to 1:1 by 
itself. No matter how the lens is moved farther from the film plane for 
close focus, there is light loss requiring increased exposure. Before 
TTL auto exposure, tables of extention factors were required.

>......Also, the vivitar 2x macro takes the normal 50/1.8 to 1:1.
>  
>
Actually, any 2x telextender will convert a macro lens that focuses to 
1:2 to one of twice the focal length, two stops slower and focusing 
directly to 1:1. The Tamron SP 90/2.5 macro lenses specifically 
recommend doing this with their matching 2x converter. They become 180/5 
with the converter and focus directly from infinity to 1:1. And it works 
very well <http://www.moosemystic.net/Gallery/Tam902.5/>.  However, the 
Oly 2xA does not physically fit the 90/2.

The Vivitar and Panagor macro teleconverters do the same thing for 50mm 
normal lenses. They take a lens like the Zuiko 50/1.8/1.4, etc. that 
normally only focus down to 1:7 and convert them into 100 mm lenses that 
focus to 1:1. They would still be a fine choice for extending the close 
focus of a macro lens, and would actually go closer than 1:1, with their 
added extention.

Moose


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