For all you consumers out there, I bought a Pentax FAJ 70-300, to see if it was as good as some of the reviews. This lens got a sort of mixed rap, but, it takes much better pictures with my k10d than the Sigma. The Sigma manual focus was tight in spots as it articulated, but it seemed to function properly.
I set up my tripod, mounted the camera and took pictures with just changing lenses, than going to my computer to 'see' if the Pentax was indeed just a cheap film lens, not digital dedicated, with no redeeming money advantage. Wrong. Although, the Sigma lens arrived in a box that was big enough to let the lens box bounce around with absolutely nothing for packing, it may have been damaged in shipment. So maybe the lens is OK, and it was damaged in shipment or???? The FA turned out much clearer and sharper than the Sigma in a fair test, but maybe the Sigma was damaged. If it was not damaged, it sure was not worth the money. I did not want to spend the money to check this inexpensive lens for optics and the electrics of the motor. I took manually focused, and automatic. I found the Sigma wanting. I try to spend my money where it goes the furthest. Get more detail about Sigma 70-300mm f/4-5.6 SLD DG Macro Lens with built in motor for Nikon Digital SLR Cameras.