The other night I was shooting BMX racing (outdoor low light). I was using my brothers Nikon D50 with a 70-300 VR and an 18-55 non VR lens. His camera had a very hard time locking on to the riders...actually to the point where I stopped shooting. What is this a function of? The camera was on auto mode and action mode (running man symbol) and auto ISO. Would the XSI and 40D act the same way. Thanks!

