I have recently gotten into birding photography and have been scouring endlessly figuring out exactly what to replace my point and shoot with so that my birding shots for nature walks don't have to be cropped to the point of losing all detail just to get a nice desktop size image of a bird. Sharpness at far distances is my main concern. I don't plan on doing anything but daytime nature photography so a large aperture is not my primary concern, sharpness is a MUST. Anyways, I am thinking about going with canon and found a "New In Box Never Used Canon 400MM F/5.6" prime for $750. It seems like a very good deal but would be quite a bit of money to spend without having ever tried it out. I've read many reviews and am still undecided on whether to get a 400mm prime or the 100-400mm 4-5.6 L IS Zoom but that would cost MUCH more and I assume that most of my images will be fully zoomed anyway. Does anyone have any experience with this or both of these lenses to offer reasonable opinions? Since I'm writing anyway, is it worth the extra money to get a 40D vs. a rebel xsi with this lens and my purposes in mind (no action photography)?