>>3998065In theory, focal length of a simple lens with one objective element would be the literal length from the objective lens to the plane of focus, so a theoretical 50mm lens would be 50mm minus the flange distance in length, a 60mm lens would be 10mm longer, a 150mm lens would be 100mm longer than a 50, and so forth. With mirror telescopes, this is exactly how it works. The distance from the mirror to the plane of focus is the focal length.
In practice, camera lenses are made of more optical elements than just the front objective because otherwise, the size would just get out of control, not just for telephoto lenses, but also for simply focusing a short to medium focal length lens from up close out to infinity. Take a look at Pentax's pancake lenses, for example. They have a 70mm lens that's shorter than their 15mm lens. From a lens size perspective alone, it makes no sense. I'm not an optical engineer, but now take a look at what a teleconverter does to the focal length of a lens and just how much size it adds and you might start to understand just how this sort of feat can be accomplished while keeping the lens size to a minimum.
So to answer your element, Nikon's optical design makes use of lens elements that require more physical space than the Canon's optical design. It's likely not for lack of trying to shrink their lens, but it's possible they just reached a physical limitation of the materials they're working with, and that's as small as they could make it while maintaining the level of optical correction they wanted.
