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Re: [OM] Internal focus

Subject: Re: [OM] Internal focus
From: "Terry and Tracey" <foxcroft@xxxxxxxxxxxxxxxxxxxxxx>
Date: Mon, 6 Jul 1998 18:01:04 +1000
--
>This one I think I know, but if I'm incorrect, I'm sure the right answer
>will come forthwith.  A "normal" lens is focused by moving the elements
>closer or farther apart by some means, generally a helicoid thread
>mechanism.  In this case, the physical distance between the rear-most
>element and the front-most element varies.  With an internal focussing
lens,
>an element, or group of elements, in between the front and rear move,
>shifting the place of focus but the physical distance from front to rear to
>not change.
>

I thought the normal lens just extended. Put a 300mm on a 25mm tube
(focussed at infinity), or wind it out 25mm, same result.

Internal focussing move elements and can focus closer (generally), but they
reduce their focal length. This isn't a problem for most situations, but
once you focus closer and on small things the focal length reduction becomes
greater. Or so I'm told.

My two lenses that modify their lens groups (Zuiko 50mm f3.5 and Tamron 90mm
f2.5) both extend.  (25 for the 50, 45 for the 90), but the elements inside
move about and things change. But it still extension.

Foxy


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