I am in France right now with a photo tour group to photograph the stunning white horses of the Camargue. This morning we put two stallions together, and the action that resulted was dynamic and very exciting. My settings for this picture were 1/5000th of a second, f/4, and 640 ISO. I specifically chose aperture priority in this case and used a large lens opening because that forced the shutter speed to be very fast. I could have used a lower ISO, but in order to guarantee that all of the water drops as well as the fast moving horses were tack sharp, I opted for a very fast shutter speed. I hand held the camera since a tripod was not necessary, and I used a 70-200mm lens to fill the frame with the action.
It was very important to watch the horses through the viewfinder. If I watched them directly with my eyes, expecting to raise the camera to my face and shoot as soon as the action started, I'd miss the best shots. Only by being constantly ready to shoot could I capture the peak of the interaction between the stallions.