Question

totally confused in sign conventional for lens

Answer 1

it's just simple.....
REMIND coordinates graph's sign....
Only in one case the signs will -ve if image is formed at other side of lens and is upward ....
in all the other cases the sign will be negative at end

Answer 2

Lens formula :

1 / f = 1 / v - 1 / u

Concave lens :

f = negative

u = negative

v =  negative

Convex lens

f = positive

u = negative

v = positive for real image ,

v = negative for virtual image .

The optical centre is chosen as the origin of the coordinate system.

With respect to optical centre :

The part which are above are positive, below negative.

The part which are right are positive while left is negative.

This is just like the normal x-axis and y-axis in the co-ordinate system .