It is possible to image with a modest single element lens. We are sorry to report that the resulting pictures are second-rate. This is because all the lenses have aberrations that deteriorate. Opticians mitigate aberrations by combining numerous lenses. Some are positive (convex) and others negative (concave) in terms of performance. In addition, some are cemented together; others are at air distances. It takes all this to mitigate deviations. However, residual errors always remain.
If the camera were equipped with a lens with a single element and focused on a distant perspective, we could take a measurement from the lens center to the image plane. This value is the focal length. In a complex lens array, finding the point for this measurement is less clear. The point we need to find is called the back node.
Opticians can and must move the position of the posterior node. Now a long lens is a long focal length. The longer the focal length, the larger the magnification. A long lens is very desirable if you are interested in sports, wildlife or the like. However, it may be that a long lens is a bit awkward.
Opticians have a trick on their sleeve that physically shortens the lens barrel. This is achieved by moving the rear node forward. If the optician so desires, a complex array of lens elements may be constructed so that the posterior node falls into the air in front of the anterior element.
Remember that the focal length is a measure measured from the back to the image plane. The advantage of such a design is a shorter, less cumbersome run length. Let me add: A real tele-design differs from the long lens in that the telephoto lens is shortened in terms of the run length.
You should also be aware that short wide-angle lenses often position the rear lens group too close to the image plane. If so, there is no room for the SLR mirror mechanism. The optician, who wants more space for the back focus removal, will move the rear knot backwards.