I am extremely impressed with this lens. There is plenty of sharpness, and the IS and lightweight to make it a formidable handholding tool. I started using it more and more over the last few weeks, mainly in response to getting the new 1DX2. As I acquainted myself with the new body, I wanted to use something reasonably sized. Otherwise, I would look like a major creep walking around Mountain View parks while toting my big 600mm. I’m sure I still look like a creep either way.
Assuming 400mm is adequate for your needs, you cannot go wrong with it. As is true for many others, the first edition of this lens was my gateway into the super tele world. It was a workhorse that yielded stunning results. Only when I began my “affair” with the 600mm II, did the honeymoon with the 100-400 end.
Over the last couple years I have been holding down a 3 body kit for wildlife: super tele; mid range zoom; ultra wide. I’d stick the 600mm on the 1DX, constantly swapping 1.4x TCs. The 70-200mm on the 1D4, occasionally with 1.4x TC. And my sidearm was 14mm/11-24mm on the 6D. This had me covered well for most of the situations I put myself in, and there wasn’t any real demand for anything in that 350-500mm range. That all began to change after getting the 7D2. This body immediately replaced the 1D4 in my kit for use with the midrange zoom. With the crop, a 70-200 became 320mm equivalent, and I was reminded of how useful those focal lengths were.
The 100-400II with 7D2 is a natural pairing, sacrificing the wide end, its 640mm equivalent reach was great. I found it too prohibitive with a 1.4x extender, locked to the center AF points at f/8. This is where the 1DX2 comes into the picture, as it has 61 AF pts at f/8.
As for how the 100-400 compares, I would argue it is quite similar to 70-200 in sharpness. Sometimes I think 100-400 is better, other times I am back to the 70-200. The IS is about the same as 70-200. When using extenders on the 100-400, I have found that on the 1DX and 1DX2, I can easily use the 1.4x TC with minimal degradation. I do not use extenders with the 7D; I think the pixel density is to high.
As long as there is sufficient light, I am comfortable using the 1.4x on 100-400 at f/8. In fact, it performs much better than I ever expected. To be honest, I didn’t have much faith it would, and it wasn’t the reason I bought. However, had I known it was this good, I would have bought the lens much sooner, and now I have to figure out how I wrangle it away from Jenna 😉