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Message: <blockquote data-quote="BackdoorArts" data-source="post: 625519" data-attributes="member: 9240">Take the camera with the lens on it, stick it at 1/250, go to a road where cars will pass by at over 50mph, put the lens at 400mm, set autofocus to AF-C with the maximum number of focus points, set the shutter mode to Ch, and start shooting cars as they go by. I suspect your keeper rate will be at most 10-15% in terms of sharpness. It's the way it is when you're panning.Then do the same with the shutter speed at 1/1600 and see the difference. This will likely tell you more about how your focus mode is set.Your 70-300mm is a baby compared to the size of the 150-600mm. Your front element is easily twice as far from the camera body and even farther when you zoom. The front element, therefore, has to move twice as fast on the Sigma than it does on the 70-300mm as you sweep, and that's going to take a lot more control.</blockquote>