Tried shooting horses running with 70-200. It kept trying to focus & wouldn't shoot

[spond]

Tried shooting horses running with Canon 6d, lens 70-200.  Pressed shutter half-way, but it kept going in and out trying to focus and wouldn't shoot.  I've taken pics of birds in flight a month ago.  I also had the same problem with my 40mm lens doing the same thing on a still portrait.  I didn't use a tripod on any of t hese shoots.

[tinwhistle]

I don't pretend to have anywhere the expertise or experience than most of you here, but what I did in this very same situation is use the Alservo, as mentioned, but I'd also set ISO at about 1000 and set my motor drive to high and let 'er rip. You're bound to get a couple super shots out of the several you run off. This is how I have shots of horses with all four hoofs off the ground. I fully expect the scoffers to take me to task, but it worked.

[cicopo]

The thing to remember when shooting "spray & pray" is that focus must be locked on your target for the first shot or the entire set will be out of focus. (based on my experiences with most bodies I've used). The auto focus rarely catches up if it wasn't locked on target to start with.