« on: July 01, 2013, 09:07:32 PM »
I owned both 1.4x and 2x extenders V.II for use on my 500/4 V.1 and 300/2.8 V.1 and when they V.III extenders were released, I upgraded both.It is worth. I have always been satisfied with my EF1.4X II but until recently the only use of EF2X II that was really worth was back in 2006 when I had used my 300mm f/4L (non-IS) with that 2X teleconverter and photographed the total eclipse.
I didn't really see any noticeable difference with the new 1.4x (perhaps just a tad in the corners) but I did find a noticeable improvement with the 2x on both lenses. Across the entire frame. I rarely used the V.II 2x because I was never really happy with the results. Now I use the V.III 2x all the time and am very happy with the IQ it delivers with both my 1D Mark IV and my 5D Mark III bodies.
If I had it to do again, I would not bother to upgrade the 1.4x extender for my use.
Just my personal experience, for what it's worth.
An improved 2x teleconverter is always useful.