Dear forum, I'm currently trying to buy some simple glass double-convex lenses with no optical coatings from China with the help of a colleague over there. I'm trying to match a lens that I already have but which is no longer readily available to me here in Brazil. The lens I have is double-convex, has a diameter of 125mm and a focal length of 318mm. I'm searching on a website my colleague sent me and have found some lenses with the correct diameter but with what appear to be magnifying strengths, eg. 3X, 5X, 8X, 10X Is there a way to calculate the focal length from these magnifying ratings? Thanks!