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RE: [OM] Extension tubes

Subject: RE: [OM] Extension tubes
From: Clendon Gibson <bsandyman@xxxxxxxxx>
Date: Mon, 24 Sep 2001 05:43:02 -0700 (PDT)
Adding extension rings does not change the focal
length of the lens. The focal length of the lens is
due to the geometry of that lens. (Which the tube
cannot change.)

The purpose of the extension tube is to change the
distance from the film plane over which the lens will
focus.

This is difficult to explain without pictures, but A
subject in front of the lens will appear in focus
behind the lens at a particular distance. The distance
in front and the distance behind are proprtional to
each other. The focal length can be used to figure out
the exact proportion. 

In order to focus on anything, the lens must be moved
in relation to the film plane so that the desired
subject is in focus on the film plane. You may notice
that even on a fixed focal length lens, say a 55mm,
the front elements moves in and out as you turn the
focus ring. (It only moves a little, so you have to
look closely.)

The default ranges on lenses go from infinity to maybe
around 3 feet depending on the lens. (Macro lenses
will get closer.)

This isn't very good fro macro work. Extension tubes
allow you to move the lens farther from the film plane
so you can focus on things that are closer to the
lens.

This does not chage the geometry and therefore the
focal length of the lens.

Hope this helps.

Date: Fri, 21 Sep 2001 14:50:19 -0600
From: "Daniel J. Mitchell"
<DanielMitchell@xxxxxxxxxxxxx>
Subject: RE: [OM] Extension tubes

> I just got a set of these - would someone tell me
how best to use 
them?
> When I put the 7 on with a Zuiko 35/2 everything
went awfully dark 
and I
> couldn't focus!

 The trick is that the focal length will suddenly be
_very_ short -- 
this
confused the heck out of me when I got a set, too. 
Hold your hand in 
front
of the camera and move it closer; it'll start being in
focus about a 
foot
away (at a rough guess). Also, the 35 will have a
shorter focal length 
than
a 50 would; might be worth a try with one of those.

 For actual lengths, try 
http://www.taiga.ca/~esif/om-sif/macrophotogroup/extension_tubes.htm

 According to that, a 50mm with a 7 tube has a focal
range of 39 to 20 
cm.

 -- dan


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