Hello, I'm looking at getting a lense for my Olympus E500, and I've found a 40-150mm lens, with either F3.5-4.5, or F4-5.6. How much of a difference will it make? I'm still learning some camera concepts, and am unsure what the difference is. I'm looking at buying it used from keh.com, where the 3.5-4.5 is $115, and the 4-5.6 is $149. What would you reccommend for a beginner? Thanks!