Sign convention for lens:



1) All distances are measured from the optical centre C.



2) When distances are measured in direction of incident ray, they are taken as positive. Here AB is an object A'B' is the image.

 


So BC is the object distance = - u (negative)

 


CB' is the image distance = v (positive)

 



3) the distance which are opposite to the direction of incident ray are negative.  [here object distance - u is negative]



4) For a convex lens the focal length is positive.


5) For a concave lens the focal length is negative.