Roughly this same shot can be taken with a D100 and screwdriver AF 70-300/f4-5.6 AF D ED. Im traveling and can't get to my archives, but I have shot MANY sheepdog trials as well as my own Labradors, and it's DEFINITELY not out of the capability of the D800.

In this case, you may have had one additional complication, which is the large amount of water in the frame - it's all at a different focus distance, and in this particular case the fact that the D800 is really fast reacting might be working against you. The focus tracking lock recommendation is appropriate for this reason.

The suggestion of faster shutter speeds is also spot on.

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Brian... a bicoastal Nikonian and Team Member

My gallery is online. Comments and critique welcomed any time!