Mammoth Memory

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.

Ray diagram showing principal focal length and image distance are nearly the same

More Info