Bruce - My wife used to shoot with the Sigma 150-600mm "C" lens, and could not get a sharp image at any focal length. I AFFT it, with the USB Dock, but that made little difference. I sent it back to Sigma with the body (Nikon D500) and they AFFT it in their lab with their priority software they claimed would adjust it better than the public USB Dock settings was capable of. It came back from them with a phone call saying that it was within specs of the lens and claimed it was as good as the other 3 similar lenses they had there used for comparison. We tried it out, only to find it was still terrible. I bit the bullet and traded it in and got her the Nikon 200-500mm Nikon lens, which performs great. So she's now happy and getting tack sharp (for a zoom) images she's now happy with.
Myself on the other hand, have the Sigma 150-600mm "S" version, and after AFFT it with the USB Dock, I am completely happy with the performance of this lens - good at all focal lengths, and much lighter than the 600mm F4 VR lens it replaced. I can even handhold this lens while in my kayak or sitting next to me in the vehicle, for those quick "grab shots" that sometimes occur. I love the zoom feature, something you don't have the luxury of with the primes. Most times in the kayak you don't have the ability to move closer or back up so the zoom feature shines! Keep in mind you're giving up a small amount of sharpness, and you also lose some degree of F stops, but all in all I love this lens! Normally I find that shooting handheld from the kayak, for instance, requires a fast shutter speed (1/1000th min.) for a larger degree of "keepers". On the tripod, obviously, you can shoot much slower shutter speeds for success. Oh, and I know nothing about the Tamron except from a few reviews that seem to say it's a decent lens.
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