Why measure Focal Length?
Unless you’re testing and calibrating lenses, I’m not sure why you’d want to know the exact focal length of a lens other than simply to satisfy your curiosity. It’s just something that some people want to know. If you pay for a 400mm lens, it’s nice to know you have a 400mm lens, not a 370mm lens I guess. In fact most lens makers will tell you that the focal length marked on a lens is +/- 5%. That means your 400mm could be as short as 380mm and still be “within spec”. Normally telephoto lenses err on the short side. It’s MUCH more likely that a lens will be shorter than marked than longer.
