So I've been taking photos for a couple years now, actually trying to take photos instead of snapshots. So far I've understood what I'm doing and can relatively grasp the concept behind things however something that I have read on here that I just don't get is how to add depth to something. Such as make someone pop out of a photo. I generally don't have this issue (just luck I suppose) but would like to understand why images seem flat or have something to them so I can avoid it in the future. Is it all just depth-of-field, or is there something I'm missing to it?