I've had 2 copies of this lens - the first, a non-DG version died eventually from sand getting into the focus mechanism and destroying it. I found that at 500mm it really needed to be stopped down to about f8. That seemed to be a good compromise between slowing things down too much and adequate sharpness. At this apeture, I have regularly had pin sharp results when printing out to A4 size. Obviously good light helps, good technique is essential, but I found it to be a good performer throughout its focal range when stopped down a bit.
Have a look at my gallery on birdforum (same user name) - the more recent shots were with a 500mm sigma prime, but the older ones are with the bigma.
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