A fast-approaching subject will test your camera and lens autofocus capabilities. The Canon EF 70-300mm f/4-5.6 L IS USM Lens performs very well in this regard - as does the 1Ds Mark III body used behind it for this capture. Running kids usually have a great smile.
Camera and Lens Settings
300mm f/5.6 1/1600s ISO 250 3744 x 5616px