What is Focal Length

A lens' focal length is a measure of how strongly it bends the light. A photographic lens (convex lens) will bend parallel light rays so that they all intersect in the same point, the focal point. The focal length f of the lens is defined as the distance from the optical center of the lens to the focal point.

The focal length is thus a characteristic of the lens only and not related to sensor size/film format or what camera the lens is mounted on. A 50mm lens is a 50mm lens is a 50mm lens.

The Field of View (FoV) of a lens, on the other hand, depends on the sensor size/film format.

[randiman]

focal length is very important to photographers, it gives you the idea of distance

[vietlh81]

This seems very basic. How to determine the focal point and optical center, especially of the wide angle lens?