Update: I've had some time w/ the 400mm f/4.5 (did some birding last weekend) - and next I'm trying out the 70-200 f2.8 S lens. The 400 is an amazing lens - purchased that under the current sale. My plan is to try the 70-120 w/ my kid on a horse indoors (poor lighting) and outdoors (better lighting (hopefully) and some distance). If all goes well I'll buy that whilst the sale is still on...