All distances are to be measured starting from the optical centre of the lens. All real distances are positive and real distances means the distance actually covered by the ray of light. So, distance of real object, distance of real image and distance of real focus are all taken to be positive. All virtual distances are negative. Distance of virtual object, distance of virtual image and distance of virtual focus are all taken to be virtual distance. This is called Sign Convention.
Focal length of convex lens is positive and focal length of concave lens is negative.
Mathematical example: If the focal length of a lens is +0.1m then what is the power of the lens?
Focal length,f = +0.1 m
Power, P = ?
We know, P =1/f = 1/ +0.1
= 10 D (Ans)