Question

sign convention of spherical lenses?With example.

Answer 1

1) distance measured in direction of  incident rays is positive and in opposite direction is negative
2) height measured above principle axis is positive and one measured below is negative

Answer 2

1) All distance are measured from the optical lens center. 
2) The focal length of the convex lens is positive
3) The focal length of concave lens is negative.  
4) all height which is above the principal are positive and similarly below the principal are negative.