Re: How we can measure exact focal length of a web cameras?

I didn't know a good answer to this question so I did some searching 
around. I finally found a web page that has a lens tutorial and a Q&A blog 
that describes the problem and possible solutions. The problem is that 
there are simple equations for computing focal length as an approximation. 
If however you need to know the exact focal length then it is a very 
difficult problem. Here are the links.
 http://photo.net/photo/optics/lensTutorial.html#part1
 http://photo.net/bboard/q-and-a-fetch-msg?msg_id=003AEP