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07-04-2011, 04:34 PM   #1
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"Focal length"

What exactly is being measured for the "focal length" of a lens?

Silly question, I know.

Thanks!

07-04-2011, 04:48 PM   #2
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Not a silly question at all. Something that I find quite interesting.

Technically . . . here is a link to the Wikipedia page for "focal length" . . . Focal length - Wikipedia, the free encyclopedia

From what I understand, if you assume parrallel lines going into the front of a simple lens, the rays of light get bent as they pass between the air to glass and then the glass to air change in medium. The focal length is where these rays of light converge. (As in the 1st diagram on the Wikipedia page at the right.)


I guess with complex lenses, unless you are designing the lens, its hard to know where the "center" of the lens is.

I once read somewhere that the term "focal length" came from Greek? and means "fire length" of a lens in which one of the earlier uses of a lens, before they were used for cameras, was to start fires. And the focal length was how far away you had to hold the lens from the kindling.

The practical definition of focal length in terms of photography are mentioned in the link above as well.

Take care,
Glen
07-04-2011, 05:20 PM   #3
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QuoteQuote:
..I once read somewhere that the term "focal length" came from Greek? and means "fire length" of a lens in which one of the earlier uses of a lens, before they were used for cameras, was to start fires. And the focal length was how far away you had to hold the lens from the kindling.
..
By far the best description I've seen!

QuoteQuote:
I guess with complex lenses, unless you are designing the lens, its hard to know where the "center" of the lens is.
The "location" of a complex lens is in effect worked out backwards from the position of the lens in its "burning positions" - there's two imaginary locations - the front and back "principal planes"- where it looks like the lens is from the front and back; these are mathematical artifices to help describe precisely how lenses work.
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