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Lester,
The f-ratio actually considers the virtual aperture of the lens which is not necessarily the physically measured aperture of the front element.
So your zoom lens will have a constant physical aperture, but a varying virtual aperture, that varies relative to the focal length, hence making it a constant f-ratio zoom lens.
I think the confusion lies in what the definition of "aperture" actually is.
In a Telescope its one and the same (front element diameter) but not so for a camera zoom lens.
I am sure others can elaborate further and better than I.
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