Viscously damped objects driven through a periodically modulated potential energy landscape can become kinetically locked in to commensurate directions through the landscape, and thus can be deflected away from the driving direction. We demonstrate that the threshold for an object to become kinetically locked in to an array can depend exceptionally strongly on its size. When implemented with an array of holographic optical tweezers, this process provides the basis for a continuous and continuously optimized sorting technique for mesoscopic objects called “optical fractionation.”