I have fabricated a few biconvex lens (12mm diameter) for an experiment and am trying to find their focal lengths. My though is to set up three parallel lasers and find where the beams converge. However, even with the smallest laser diodes I could find, three beams surpass the diameter of the lens. Would two beams be sufficient? Or does anyone have an alternative method of experimentally finding the focal length of small biconvex lens?