A little background first, I have a rebel xs, a limited budget & want a decent telephoto for shooting sports car racing. When I was at the Rolex 24 this year 300mm (plus the 75-300 kinda sucked anyways) just didn't feel like enough and with only 10 megapixels you can only crop so much. So I've been looking at the under $1000 telephotos and the Tamron caught my eye. My concern is that with a 6.3 aperture @ 500 that the rebel won't be able keep up with AF even in bright sunlight. Anybody have any opinions or other options?