Especially if you want a good aperture on top of fast focusing. Not to mention that premium f2.8 telephoto lenses easily weigh upwards of 1.5kg (and easily exceed $2000). Not very practical. Which is why the Pentax 55-300 is pretty much ideal - it's compact, has a reasonable aperture while being lightweight and affordable, and it has an excellent focus motor.Hmm true, lenses for fast moving things like sport won't come cheap..