How is a zoom aperture fixed?

Why is f-stop not based on actual aperture size?

• Recently, I learned that f-stop is actually based on the entrance pupil size (the size of the pupil as it appears through the front of the lens where it may look magnified), not the true aperture size. Why is this, isn't the absolute size of the aperture opening what matters, not how it looks through the lens? Also, why is it that fixed-zoom lens cameras don't go past f/8 or f/11, while others do?

• Answer:

  The aperture is measured as a fraction of the focal length as the focal length determines from what angle the lens gathers light. Imagine a wide angle lens that gathers light from an angle of view of 90°, and compare it to a telephoto lens that only gathers light from an angle of view of 5°. The former will gather much more light, won't it? It'll get all the light that the latter gets, but also light from an additional huge area around that from which the latter gathers light. For this reason the focal length - which correlates to the angle of view - is included in the measurement of the aperture opening. Otherwise a lens with the same absolute aperture opening will gather different amounts of light depending on the current focal length.