I am wanting to purchase a zoom lens. I see one which might be 24-120 mm f/3.5-5.6 Another might be 24-85 mm f/2.8-4.0 I understand the 24-120 and 24-85 zoom but I don't understand the significance of the f stop. What do I need to know to understand and make a more intelligent decision in the purchase of a zoom lens. Sorry for being so stupid. Thank you