>If I switch to an f/4 lens on the same camera, either I will >have to lower the shutter speed to 1/80th or raise the ISO to >102,400. If I choose the first, I will get more motion >blur in the subject, and if I choose the second I will >get reduced dynamic range and increased noise. It doesn't >even matter if the lens itself is better; subject movement and >digital noise are outside the control of the optics.
There are so many posts on the subject, point & counterpoint, but I think I at least understand the points in this post of yours. I don't have either lens. If someone wishes to send me one of both, I will test the heck out them (might even lose the return address).
Okay, Brian, here is what I understand. I understand that there are very real & legitimate needs where larger aperture is absolutely necessary.
What I don't understand is your motion blur argument. My understanding was that the improvement in VRIII would gain at least one, maybe two stops over VRII. My thought from this is--from your example--that there should be no more motion blur between VRII @ f/2.8 than VRIII @ f/4 with all other settings/conditions being equal. I will give you that DR & Noise could well be affected.
Now perhaps the thing that I don't understand is whether there is a difference between motion blur caused by the subject's motion, and that caused by movement of the camera; and by how well VR handles either of those.
I am not trying to be confrontational nor argumentative. I am just trying to understand. Thanks for clarifying any of this for me.