Practical ways to find the principal focus length – Pins
To find an easy and simple way to measure the focal point of a concave lens, take the concave lens outside on a clear day.
Place the lens against the edge of a flat piece of board. Allow the sunlight to pass through the lens but note it is critical to this experiment that the sun is in line with the principal axis of the lens.
Put two pins in the board perpendicular (at right angles) to the board and in the path of the light and trace their shadows with a pen on the board.
Take this board and project back the two traced shadow lines. We can consider the sun's rays as all being parallel so we know where the shadow lines project back will be the principal focal point.
Make an allowance for the lens centre and if you measure the distance between the lens centre and the principal focal point, you get the principal focal length.
In ray diagram format this would be: