As I read the post and replies on the site I find myself confused. If someone could help explain this to me that would be great. 1. fast maximum aperture - What exactly does that mean and how to you read apertures. Like F3.5 isn't as good as F1.4 2. Is aperture the most important thing to consider when buying a lens? 3. External Flash - Do i need one? I have the XTI with built in flash but would advantages would i get with a external flash? Thanks!