How
are the aperture settings calculated ?
To
many photographers the f stop number system that relates to the
aperture seems to have no sense, they seem like a strange set of
numbers that are hard to learn and sequence in the opposite direction
to our logic.
When
they look at a full set of aperture numbers,
they may notice that every second number is twice or half as big,
ie f11, f22., and they may know that the difference between each
setting is half or twice as mach light. ie. f11 lets in twice as
much light as f16.
But
it happens that the f number is designated by the number of diameters
of the aperture for that setting  which can be divided into the focal length of the lens.
Open
or wide apertures 
stopped
down or closed apertures 
f stop
setting

f l.4

f2

f2.8

f4

f5.6

f8

f11

f16

f22

f32

f45

f64

So
the diameter of an aperture of f2 divides into the focal
length of the lens by exactly 2 
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And
a diameter of an aperture of f4 divides into the focal length
of the lens by exactly 2.
f16
would divide into the focal length by 16 and so on.
Because
f2 is a larger diameter or opening than f16 this is also
a good way to which aperture is smaller or larger than another. 
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