A sort of technical question on focal length

[Rebel]

I've often wondered how they come up with the measurement in millimeters when referring to focal length. Lets say a 200mm lens, what is 200mm about it? Could someone enlighten me in lay mans terms.

Thanks in advance!

[Olegis]

Here you go - http://www.dpreview.com/learn/?/Glossary/Optical/Focal_Length_01.htm

[EXA1a]

I've often wondered how they come up with the measurement in millimeters when referring to focal length. Lets say a 200mm lens, what is 200mm about it? Could someone enlighten me in lay mans terms.

Thanks in advance!
The basic principle in a simple experiment:

-Take a magnifying glass
-Let parallel light rays shine on it (e.g. the sun, light comes almost parallel)
-Hold a sheet of paper behind the magnifying glass
-Adjust the distance so that the light is being bundled to a tiny little spot (focal point) on the paper which can burn a hole in it
-The distance between the lens and the paper represents the focal length

However, it becomes a bit more complicated if you take a lens with more than one element because you don't know the exact point within or even outside the lens from which to measure the distance, e.g. there are 8mm lenses with a focal point 40mm behind the rear element of the lens (retrofocus principle).

--Jens--

[Rebel]

Fabulous!

Thanks for clearing that up.