If the distance of object and image are not measured from optical centre (C), but from first and second principal foci respectively, then newtons formula states f1f2=x1x2 where, x1 = distance of object from first focus F1. x2 = distance of image from second focus F2 If medium on either side of lens is same, then −f1=f2=f So, Newtons formula takes the form, x1x2=−f2