Hi, Alan, please, could you compare the shifting movement of elements within the 100-500mm and 100-400mm lens barrels? I was startled by how, when the lens is not attached to a body, or when it is and IS has been disabled, the inner elements seem to really flop about! For a second my heart dropped--but then I guessed it just had to be that way for IS. Sure enough, enabling IS not only reduced the shifting, but I could watch the IS working as the elements shifted to react to my movements.If a lens does what you need it to do, then it is suitable for you. I am always looking for light lenses, the lighter the better, and actually bought the Tamron 100-400 because of its size and weight, and a good review. But, the AF wasn't up to it for flying birds and I sold it after a month at a loss. I also like a lens that it is sharp at the edges because when following erratically flying birds, there are often good shots when the bird is awkwardly at the edge of the frame. So, my requirements are for fast and accurate AF and sharpness all over the frame. The 100-400mm II fitted the bill and the 100-500 I have just bought not only has excellent AF but the best edge performance outside of a prime. None of these lenses is flawlessly sharp - you'll see the difference against a good prime. It's horses for courses.
My wife briefly couldn't see it until she held it herself and in pretty good light looked straight down the front of the barrel and then gently tilted the lens in different directions. When she did, her eyes got very wide.
I checked my Rf 70-200mm and see the same type of shifting, but not nearly as dramatic. Of course it is shorter. (How did I miss it before?)
So, just curious...Does the 100-400mm have the same visible shifting about of the inner elements? And the old 70-200mm? If so...HOW DID I MISS IT FOR SO MANY YEARS???