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Jay,
It's the size of the main mirror that defines how much light gets in.
So, both the 200mm f5 and the 200mm f4 will receive the same amount of light.
Simplistically, the difference is that the f4 "concentrates" that light into smaller images, hence they can look brighter.
"Faster" optics like an f4 put demands on the eyepieces - better eyepices are required, which usually means $$$
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