I'm getting a Canon Digital Rebel XTi and am researching lenses, and I'm not sure what the advantages are of fixed focal length lenses as opposed to zoom lenses. It seems like zoom lenses would be more practical because they can be used for a wider range of distances without having to change the lens (and also since I'm a college student and can't afford more than one semi-expensive lens at the moment). I've been reading some reviews of both types, but I don't really know which would be a better investment. Any advice?