I think it's more of a question that you have to ask yourself. I want to acheive this effect, to get a tweety bird at 40 paces I need this focal length or have "this" amount of money. There are several other reasons to ask questions about lenses, but I dare say that one source is not available. It takes research. There's a ton of lenses out there and you need to figure out which direction you are leaning towards.