Hi! I'm looking at getting another lens to compliment my kit lens (18-200mm tamron) and I'm after something for action photography so was thinking about a fast prime. Could someone please explain what the advantages of getting an f1.4 over a f1.8 would be? Is there a large average price jump between the two, and do you think the price jump is justified given I'd like to do action photography? I shoot with a D50. Thanks for the help.