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Re: [OM] Questions, questions...

Subject: Re: [OM] Questions, questions...
From: "John Petrush" <jpetrush@xxxxxxxxx>
Date: Sun, 5 Jul 1998 21:41:02 -0400
Ingemar Uvhagen <ingemar.uvhagen@xxxxxxxxxxxxxx> wondered:
>
>First the type of glasses. <.......snip......>
sorry, but I just can't help much on glass - I too would like to know more
about that.  All I know for sure is when the glass is special enough to get
an acronym, the price of the lens in question goes **way** up.

>Then I have a wonder about a thing called Internal Focus. What is that?
>(Hmm, I thought all kind of focus was "internal"...) For lenses without
>so called "internal focus", what kind of focus do they have?
>
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.

John P




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