djam

I am doing an experiment to calculate focal lenght of a lens. From a graph of 1/d0 against /di How will I find the focal lenght The points which i have do not touch the x or y axis so I am not sure if I need to extend my line. Also If i know the uncertainty in the distance to the object(do) and distance to the image (di) how can i find the uncertainty in 1/d object and 1/d image Thankyou