I think you see chromatic aberration which is blue not grey. To avoid focusing problem in situation like this I would use the manual focus.
a3dtot wrote:
Excellent and thank you. I have new, or at least new to me equipment and learning the new zoom lens. I have a Canon 5D MKII and using Sigma 70-300 lens. So the focus problem I understand. Is there some greyish coloration in the foreground limbs or are they just black?