Tamron's 70-210mm F4 Di VC USD is a lightweight, weather-sealed and cost-effective alternative to first-party constant aperture zooms. It's also likely to be used for everything from portraits to action, and we figured we should test out its autofocus chops by aiming it at some muddy motorcycles tearing around a track.

Check out how Tamron's DSLR telezoom does with these fast-moving machines in our updated sample gallery.