Lens designers have a few tricks up their sleeves. As an example, zoom lenses change focal lengths but the aperture diameter does not physically change with the zoom. This is an oddity because as the focal length changes, so does image brightness. In fact, if the focal length is doubled, image brightness falls off 4X. Conversely, if the focal length halves, image brightness will increases 4X. How do you think the optician compensated and maintained the exposure? The front group of lens elements acts as a magnifier. In other words, the diameter of the iris as seen by the outside world appears expand and contract with the zoom when in fact it is fixed as to diameter. It is the apparent diameter of the iris and not the actual diameter that determines the light gathering power of the lens. Note: most inexpensive zooms give up the ghost as the user zooms towards max magnification. A more costly zoom will likely maintain a constant exposure throughout the zoom.