Spot with Expansion AF for Track?
Looking to upgrade my 40D for track and basketball. This spring I used the 40D with a Canon 70-200 2.8 non-IS but struggled with focus. Consistently just behind the runner (ie-trailing foot tack sharp, body slightly blurred). Looking at the 7D or 70D. I'm just a dad who is improving in photography but with a long way to go.
So, what I'm looking for is a camera with all the AF help I can get AND the ability to micro-adjust focus for this lens. Will be shooting sprints with runners moving toward the camera.
Question: It looks like the main thing I sacrifice regarding AF for sports with the 70D is spot with expansion. I'm not sure if this is something that should be a deal-breaker given the type of shot I'm trying to get. How important should this be to me? It's difficult keeping the dot on a fast-moving runner, so it seems like it would be helpful. However, there can be runners very close to my target in the field of view, though slightly behind my target. Will it get fooled enough by that situation to wash out the gains from the AF expansion? Would it cut my missed focus total by an appreciable amount?
If AF is relatively equal and Expansion isn't of noticeable help in this situation, I will opt for the 70D to give me a little more room to push ISO during basketball season. Wasn't sure which to prioritize, the AF Expansion or the extra ISO.
Thanks for any input!