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Each f-stop doubles or halves the exposure time, but the difference between 1.8 and 1.4 is actually only 1/2 a stop, not a whole stop. 1.8 exists because of some weird historial reason. Also the f-numbers are rounded up or down - 1.4 is actually 1.375 etc.
The 'usual' camera f-stops are the following, and I've put in the 1.8 for comparision. The second collumn is the time - say in seconds - that would give the equivalent exposure, disregarding film reciprisosity failure (how do you spell that?) and noise effects etc.
f .......... seconds
32.......... 51200
22.......... 25600
16.......... 12800
11.......... 6400
8........... 3200
5.5........ 1600
4.......... 800
2.8....... 400
2.......... 200
(1.8)..... (150)
1.4....... 100
1 ......... 50
0.7....... 25
Often the main reason for having a 1.4 instead of a 1.8 is to show off a bit. The extra bit of glass looks nice... Having said that, often the lens manufacturers will have better optics in the faster lenses, hence the big increase in price. It's really the law of diminishing returns - or Paranti's Law (?). You get 80% of the results for 20% of the effort (or cost). but sometimes 80% is not enough....
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