You can find the negative focal length of a negative lens by placing a positive lens of equal or stronger focal length and seeing what the resultant focal length is. If the focal lengths are the same, then the lenses will effectively become a 0 power piece of glass. The focal length is this formula: 1/(fl of neg. lens) = 1/(fl of sum of both lenses) - 1/(fl of pos. lens). Thus you see that if the FL of both of the lenses together is infinite then the value of the pos. lens is the value of the negative lens with the opposite sign. Needless to say, if the pos. lens isn't at least as strong as the negative lens then the formula doesn't work nor can you get a focus of the source. It is also assumed for the best results that the lenses are close to each other for this test.