I've noticed that zoom lenses have a range of focal length and f-number. So far so good.
However, keeping the aperture size constant, shouldn't the f-number decrease as much as the focal length increases?
Say the focal length is 18-200mm and the f-number is f/3.5-5.6.
At 18mm and f/3.5, the aperture is 5.1mm.
At 200mm and f/5.6, the aperture is 36mm.
Why the change? Why not upen up to 36mm even at a wide angle?
Is this because the aperture stop isn't actually at the first lens, but deeper inside the camera and moves less (or sometimes not at all) with the zoom? In that case, why build it in such a wasteful way?