Focal Length Explained: A Beginner's Guide – Camera Harmony
Approximate ways to find the principal focal length
What is Focal Length in Photography? - Focal Length Explained
The value of the focal length of the lens is equal to the value of the image distance when rays are (A) Passing through optical center(B) Parallel to the principal axis.(C) Passing
Determination of Focal Length Of Concave Lens Using Convex Lens - BYJU'S