What is Lens Formula?

Measure all distances from the optical centre of the lens, with the direction of the incident light as positive. A convex lens has a positive $f$ and a concave lens has a negative $f$. The ratio $\dfrac{v}{u}$ gives the magnification. The sign of $v$ tells you the kind of image: positive for a real image on the far side, negative for a virtual image on the object's side.