A lens' ƒnumbers depend on the focal length of the lens and the diameter of its aperture.

HALF STOPS
This comes from our "Department of Knowledge You May Never Use."
If you want to know what the exact halfway point between two ƒstops is, add the previous two ƒstops together. For example, to find the halfstop between ƒ4 and ƒ5.6, add 2 + 2.8, and you will get 4.8. This works for all ƒstops. For instance, the halfstop between ƒ8 and ƒ11 is 9.6, derived from adding the two previous ƒstops (4 + 5.6) together.
ALL THE STOPS THERE ARE
We'll make it even easier for you, and give you the entire scale of full and halfstops from ƒ1 to ƒ90. We don't know what you'll do with the information, but here it is: 1, 1.2, 1.4, 1.7, 2, 2.4, 2.8, 3.4, 4, 4.8, 5.6, 6.8, 8, 9.6, 11, 13.6, 16, 19, 22, 27, 32, 38, 45, 54, 64 and 90.
HOW A LENS' ƒNUMBERS ARE DETERMINED
A particular lens's ƒnumbers depend on the focal length of the lens and the diameter of its aperture. The focal length of the lens divided by the lens aperture's diameter is an ƒnumber. For example, a 50mm lens with a 25mm aperture is ƒ2, whereas a 200mm lens with a 50mm aperture is ƒ4.
