What is the difference between the effective focal length and the back focal length?

The effective focal length of a lens is the distance from the primary plane of the lens to the focal point, when focusing light that is coming from infinity. The primary plane is determined mathematically and is located at a point within the lens, therefore making it extremely difficult to work with its position for mounting purposes. The back focal length is the distance from the back of the lens to the focal point of the lens when focusing light from infinity. The back focal length is much easier to use when determining a mounting position for a lens. See the accompanying diagram below: