# How do I determine the lens to set the focal length of a converging or diverging beam?

I have a gaussian beam going through two lenses. The first lens is focusing a collimated gaussian beam to a focal point either before or after the second lens.

What formula do I use to get the focal length (the strength of the lens) of the second lens such that the beam, after going through both lenses, focuses at a point "x" after the second lens?

• I'm setting up an experiment but have little knowledge in optics – Goldname Jul 1 '18 at 17:33
• – HiddenBabel Jul 1 '18 at 18:15
• For the formula, does the total focal length calculated factor in the distance between the two lens? – Goldname Jul 1 '18 at 19:13