If you took this shot with a DSLR, then if the focal plane was out, you would see it by either back or front focus of the numbers on the buoys. Have you seen this? According to the depth of field calculator, the dof of a 400mm at f/5.6 on a FF is 110-144m at 125m and 200-340m at 250m. The DSLR AF must be considerably out of kilter not to be in these dof zones.
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