I'm using the XSi kit as it says in my sig. I've owned the camera for about 9 months now and I think I understand all of the functionality. I realize that at a "fully open" (so low f-spot right?) aperture I get the most shallow depth of field and most light, and vice versa. What I'm curious about is, how do I make a decision between let's say 7 and 29? Any tips, or general rules? It gets more difficult to see the differences in the viewfinder after around 10, so I'm wondering what the high f-spots are for? Thanks, and pardon my noobish ways.