Please forgive me if this is a stupid question. I am still a newbie concerning manual photography. I am thinking about purchasing my first dSLR and I have a question or two 1. When choosing lenses, I have noticed that each lens has what I assume is a max F/Stop # with it. When connected to my camera, will the camera just 'know' that only a certain level of aperture can be used, or if I choose a lower F/Stop than the lens allows, what happens? 2. I am struggling with the choice between the Canon XT and the XTi. I would love to save $200 and purchase the older XT, so I can put the $ toward lens purchases. But now I am nervous about dust, and perhaps I should go for the XTi. I live in a house with hard wood floors, and forced hot air heat, and this place seems to have a never ending supply of dust. What do you all think? Thank you very much for any helpful advice. I am a long time MacRumors lurker, so I don't post often, but I read everyday!