Finding the image made by a lens by a scale diagram

The image can be found by a diagram as shown below.
The object and focal length are drawn to scale and then three rays are drawn from the object:
(a) the axis;
(b) a ray from the top of the object parallel to the axis (A);
(c) a ray from the top of the object passing through the centre of the lens (B).
Two examples of this are shown below.


Convex lens

After passing through the lens ray A will go through the principal focus (F).

Concave lens