Is it possible to calculate the focal length of a refactor I built? I am trying to build a very low cost refractor telescope. It's for a project, so I need to be able to provide "data" about the telescope. But it seems I can't calculate anything (magnification, to be more specific) without knowing the focal length of my telescope. However, because I built it myself, I do not know what my focal length is. How can I find that out, or how can I calculate the magnification?

On another note, what is the best combination for the best magnification? I am not looking for anything great, just the best magnification possible.

I can have a 50mm lens with a fl of 50cm, 30cm, and 20cm and a 38mm lens with each of the same focal lengths. Which is the best pair?
The focal length of a telescope is the focal length of the lens at the front of the scope. In your case it's either 50, 30, or 20 cm.

To magnify the image formed by one of those objective lenses, you need an eyepiece. None of the lenses you listed will work as an eyepiece unless you want a magnification of 2.5 (50/20).

Eye pieces generally have a focal length of 25 to 30 mm or less. To get a quality image, you should buy a used eyepiece.