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What is "focal length" REALLY?

Started May 22, 2009 | Discussions thread
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Dario D
Forum MemberPosts: 79
What is "focal length" REALLY?
May 22, 2009

I always come across different definitions of "focal length". I was thinking it meant the range of focus (ie, a lens that can focus on something 1" away has a minimum focal lengh of 1"), but then the guy at the camera store said my 18-200mm lens has a max focal length of 200mm, and NOW, I looked it up via Google (a search for "define: focal length") and got BOTH those definitions.

( http://www.google.com/search?hl=en&q=define%3A+focal+length&btnG=Google+Search&aq=f&oq= )

1 - The measurement that determine the magnification and field of view of a lens. Expressed in mm.
2 - focal distance: the distance from a lens to its focus

3 - The distance between the center of a lens (the principal point) and its focal point.

What's the deal? What does it really mean?

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