1 stop reduction in exposure = 121L = ND2
2 stop reduction in exposure = 121M = ND4
3 stop reduction in exposure = 121 = ND8. This is a "hard" cut off from filtered to unfiltered. In other words not a gradual transition.
3 stop reduction in exposure = 121S = ND8. This is a soft, or graduated cut off from filtered to unfiltered. Useful where there isn't a straight horizon to define where the grad should end. Which is pretty much all the time.
121F - the enigma. This is indeed a graduated filter that covers about 3/4 of the whole thing. Exposure wise, it goes from about 4-5 stops in reduction at the top to nothing at the bottom. Useful for when you have something above the horizon that would show a visible filter graduation. EG, a building, cliff, etc. How do you expose? Trial and error to work out how it behaves.
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