Hi everybody and sorry for initiating such a ridiculously beginner level thread.

I am confused after reading some documentation regarding lens systems as I have very poor knowledge and background.

Here they are:

1) What is the meaning of the focal length of the system?
2) what are the field angles and why are there 3 separate value in some papers and helps (e.g. 0,10,20)?
3) Why is wavelength mentioned and there are 3 separate sets of rays focusing differently (mainly in red, green and blue colors)? does that mean the image will have different focus for different colors of the source?
4)what is paraxial F/#?

Thanks in advance for any helps and responces.