I have never been happy with the IQ of any extender on the zoom lenses. On the supertelephoto's they perform very well with the 1.4x, but IMO there is to big of a IQ drop with the 2x even on the supertelephoto's.

I bought the III generation 1.4x to use with my 500 and 300mm. I sent it back. The generation II was made for the older lenses and on mine it performed better than the new version. If you had gotten the new 70-200mm F2.8 II I would say go with the new extender there are allot of raves and good reviews on the combination. I would speculate from the experience I had with the III that with the older 70-200mm F4 model you could stick with the cheaper II versions and get just as good IQ.

Maybe some here have tried the III with this lens. I think your best bet would be to review the charts:

http://www.the-digital-picture.com/R...mp=7&APIComp=0

I found the charts to be misleading. The III sharpened the corners but softened the middle. The other problem is that the AF which was dead on with the II version had to be adjusted with the III combination.

good luck