The lens designers have some tricks up their sleeve. As an example, zoom lenses change focal lengths, but the aperture diameter does not change physically with zoom. This is a rarity because as the focal length changes, so does the brightness of the image. In fact, if the focal length is doubled, the brightness of the image is reduced by 4X. Conversely, if the focal length is reduced by half, the brightness of the image will increase by 4X. How do you think the optician compensated and maintained the exhibition? The front group of lens elements acts like a magnifying glass. In other words, the diameter of the iris seen by the outside world seems to expand and contract with the zoom when in reality it is fixed in terms of diameter. It is the apparent diameter of the iris and not the actual diameter that determines the power of light capture of the lens. Note: the cheapest zooms leave the ghost as the user approaches the maximum magnification. A more expensive zoom will probably maintain a constant exposure throughout the zoom.