Determining the focal length of a camera

[georgiosd]

Hello everyone,

I need to know more about the focal length of a camera but all the information I could find on Google was regarding lenses, especially photography lenses, in general.

What I wanted to know was:

a) Most importantly, how to determine the focal length of a camera (and how hard/easy it is)?

b) When/how does it change?

Thank you
Georgios