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When I get back home I'll post a text file of some of my lesson plans from my teaching days. Ibby's got a pretty good explanation but has crossed a couple of minor wires.
The simplest explanation for anyone with simple math skills is
Exposure equals intensity * time.
E=I x T Just like any equation you can shift the parts around.
f stops are a fraction. The diameter of the opening in the lens over the focal length (the distance from focal point of the lens to the film plane when the lens is focused at infinity) of the lens.
eg If the diameter of the aperture or opening is 25mm and the focal length of the lens is 50mm then you are at f:2.
Another way of saying it is f: (in this case focal length 50mm)
50mm:2 = 25 mm
That is why when the f: number gets numerically larger, the opening gets smaller, because the f: number is just part of a fraction.
50:25 = 2mm.
It doesn't surprise me that you don't understand exposure since you live in a city which has no light to speak of. (nyuck nyuck)
But An example of E= I x T is getting sunburned.
Anyway, more later
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