I HATE having to decide. I'm heading to maritime canada in a few weeks and know I'll have to make some decisions there, and I'm tortured by it. To make matters worse, my friend will be renting a plane to do some aerial shots of Prince Edward Island, and I've never done that, so must I take the whole line up? Decisions, decisions, ugh. To answer your specific question, HOW do I decide? It depends on the situation of course, but on a paid gig, I take my rolling suitcase and bring it all, giving me a range from 14mm to 400mm. If I'm just going out to see what I can see, my 50mm is what I bring. My camera is heavy enough without the burden of a longer focal length, and while I will typically WISH I had brought something additional, I make it work.