From what I've heard from other users the Sigma 150-500mm isn't as sharp as it could be. The Sigma 50-500mm F4-6.3 also called the Bigma is a better lens and well regarded by users and I've got one of those Nikon fit. It could be the OS that may be causing the problems, so suggest you switch it off on any thing over 300mm. As your other lens gives great results it's not a camera problem. You need to brace yourself whilst taking the shot, with one foot placed infront of the other and gently press the shutter release, as stabbing it can cause camera shake at that distance. Also, make sure the camera and lens are well balanced in your hands before taking the shot. Ghosting can be caused by slight movement of yourself, when taking the shot or subject movement, if the speed is too low. Use a speed of 500s or greater may solve this problem but it means the ISO will be quite high which could lead to noise in the photo. Also, lenses at this length work best in bright sunny conditions, as the light drops on cloudy days or dusk it will struggle to focus on subjects that are low in contrast.
