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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