Focal length in x and y, stored as a 2-element vector [fx fy] in pixels.

fx = F * sx

fy = F * sy

F is the focal length in world units, typically in millimeters. sx and sy are the number of pixels per world unit in the x and y direction respectively. Thus, fx and fy are in pixels.

The focal length F influences the angle of view and thus affects the area of the scene that appears focused in an image. For a fixed subject distance:

A short focal length offers a wide angle of view allowing to capture large area of the scene under focus. It emphasizes both the subject and the scene background.

A long focal length offers a narrow angle of view, thus reducing the area of the scene under focus. It emphasizes more on the subject and restricts the amount of background from being captured.

You cannot set the value of FocalLength after you create the cameraIntrinsics object.