Aperture Help! Difference in Lenses?

Question by Tom G: Aperture Help! Difference in Lenses?
I am an amatuer photographer, and am still getting to grips with everything

I know that aperture affects the amount of light entering the camera, but what I want to know is the limits

For instance, my Dad recently purchased a 70-200mm F/4 zoom lens for his Canon.
This means that the minimum F stop is 4 right?
But that mean that it is the only one he can use? What if he wants to use higher than that? Can he?

There was a 70-200mm F/2.8 zoom lens as well (but it was almost 4 times the price)
I again assume that this lens can also shoot at f/4? Or if you want that do you have to go and buy 2 separate lenses? Equating to almost £2000??

Lastly, on some zoom lenses, the f/ value changes, why is this?

Thanks
Tom G
Thanks for the great answers, that is what I thought, though I was scared I had got it wrong!

Best answer:

Answer by Bill M
Yes, you can adjust the aperture on almost every lens that exists. The f/value changes on some zoom lenses because the focal length changes.

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