# 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?

• +1 My Unitron 2.4 inch telescope came with something like 25mm, 18mm, 12.5mm, 9mm and 7mm so I think "Eye pieces generally have a focal length of 1 cm or less." is a bit on the short side. – uhoh Mar 6 '18 at 2:51