Hello there,
Thank you all for these very usefull information.
As everybody who uses this lens I am very happy with it; I still cannot believe how sharp it is, even with the 1.7 wide open. I have made pictures with this combo where while zooming in the pixels became visible before the picture became unsharp (isn't this called oplossingsvermogen van de lens or something)?
Off course I know that the 1.7 is supposed to be less sharp and to focus less accurately than the 1.4. However as I wrote above, my experience with she sharpness of the 1.7 is contrairary to that, so I wonder if there is such a difference in focus speed and accuacy. The only way to find out is buying one and test it myself, which is probably what I am going to do first. It will win me half a stop compared to a 300mm f4 / 1.7 combo (and cost me 80mm and half a stop compared to the 300mm f2.8 ) but is a cheaper - and not less important - easier to carry solution. I believe that the 2.8 can be hand held, but as I do some travelling I like the way I can put all this in a normal size backpack...
By the way I use it with a D90, and I was thinking about buying the 300mm 2.8 af-s II. The first one was I believe the first Nikon af-s, the second one had the same optics but was lighter/less well built, then came the VR I and II's. This lens can be found used for around 2200 euro's. Sounds like I reasonable amount of cash but I prefer thinking in depreciation, not in purchase price which makes the hobby a lot bearable haha
Other thoughts are still welcome, esprecially I would like to know if anybody has ever tested both TC's on the f4!
Good hunting!