Zellmer normally uses a Canon DSLR with a 70-200 mm f/2.8 zoom lens to shoot sporting events, but recently tried some shots with the iPhone 5s burst mode that look almost pro quality. He notes that the "lack of a big zoom requires quite a bit of moving around, something that is not always possible in an active sports venue."
Rather than use the big glass to shoot a tennis match, Zellmer got about 20 - 25 feet away from the players, used the digital zoom of the iPhone 5s, and then used the burst mode to capture a series of images. The result, seen in the image at the top of this post, shows just how well the 5s works for action photography. The tennis player is literally captured in the air, the ball frozen in place just before it is hit with the racquet.
Zellmer speculates that some current technologies (including light field photography) applied to future generation iPhones could be just what is needed to make Apple's smartphone the go-to camera for sports photographers.