I've rented a Canon 70-200mm f/2.8L IS (which basically means, big zoom, good in low light, best quality glass possible, Image Stabilization - thingy inside lens that compensates for my slight jiggling of lens). IS is quite necessary for zoom lenses. Holding a camera steady in low light for a clear picture is hard enough. But if you're zooming way far away, then the slightest movement will make a bigger blur, than if you were taking a pic of something not so far away.
We took a special trip to Golden Gate Park just so that I could try the lens out on Charlotte.
Next - action shots! I enticed Charlotte to fetch a stick, and she did so with a peppy prance. Not as clear, but then this is a fast moving target. The first picture is better lit - even on cloudy days, when the light seems to be even all over, it's better if the subject faces where the sun would be in the sky. The first is front lit, and the second, back lit.
I should point out that the full length of my camera with this lens and lens hood on it 3/4 the length of my arm. It's also very heavy. But it's so much fun!