Minnesota_Steve wrote:Interesting, my 500 F4 does not struggle with the TC-20E III.I think it's a VR thing, I mainly shoot handheld. A friend of mine has the 600 f4 VR and he compared the VR responsiveness of both using the 2x TCIII and found that the 600 was much quicker to get VR working than the 500 and this may be the problem I am having. However, I have used it with great success at times.D800 + 500mm f4 VR + 2x TCIII handheld,1/250s f/13.0 at 1000.0mm iso2200Yes, with the 1.4 TC - it is sharp wide open at f/5.6.  With the 2X TC, I stop down 2/3,but great images.