Approximate ways to find the principal focal length
An approximate but a poor way to find the principal focal point is to take a very distant object and find where the image is brought into sharp focus on a sheet of paper through a convex lens. This is strictly only an approximate measure of the principal focal length because it is actually the image distance (`di`). The image distance (`di`) will always be greater than the principal focal length but when the distant object is far away they become closer.
