When the Rebel XT was released with 10MP, there was an immediate reaction in the DPReview 10D/20D/30D forum. How could Canon put a 10MP in the entry level body and leave the 30D stranded with just 8MP? In time, it became clear that the extra 2MP adds very little to resolution while the decreased pixel size, or rather sensel size, adds a touch more noise at high ISOs. Needless to say Canon puts a number of capabilities in the X0D series that don't make it into the Rebel series. Higher frames per second in burst, better autofocus, more metering modes, larger, brighter viewfinders, and more robust shutter mechnisms to name just a few. Yet with each generation, the feature gap between these classes shrinks. With the original Digital Rebel, Canon literally handcapped the camera in software in order to preserve 10D sales. The latest Rebel gets spot metering, a sophisticated AF system, larger OVF than before (though still mirror rather than prism), and even highlight tone priority! All this plus 12MP (the 40D has 10MP) and contrast detection AF in live view! Thus the 40D user outcry at having been jipped in the megapixel race was expected and even somewhat understandable. What's interesting is that just as multiple threads are popping up in the 40D forum, with one new 40D user actually crying about it, prospective Rebel XSi buyers are complaining about the pixel packing in the Rebel forums. If we assume that the Rebel Xsi and 40D have similar sensor technologies and equal quality low pass (AA) filters - and neither of those is truly a safe bet at this point - then a couple conclusions follow. In good light, with a good lens, the Xsi should be capable of capturing a touch more detail than the 40D. In low light at high ISO, the 40D ought to fare a bit better. Neither of those advantages will be very significant, and the difference in dynamic range corresponding to such a small change in sensel size is negligible.

Assuming Canon hasn't put a crappy AA filter in the Rebel XSi, it should be pretty sweet for focal length-limited shots of distant birds in flight. If light is good, an XSi user should be able to get significantly more pixels on a distant bird than a 1D III user with the same lens. The particular lens that springs to mind for this application is the 400mm f/5.6L, which should pack enough resolution to take advantage of each one of those little pixels. Click here for a great illustration of the "crop factor" at work, and keep in mind that the advantage shown for the 20D compared with the 1D II on that site should be even greater with the XSi compared with the 1D III. Highlight tone priority is another welcome addition that should come in handy with bird photography, where white feathers often present an exposure challenge. I think the XSi is exactly what I've been waiting for in a wildlife camera.