I shoot most field sports in both day and night conditions. When I acquired my 400mm F2.8 IS almost 11 years ago, I purchased a 1.4X and 2.0X Externder, both version II. At the time I was shooting a 5D with 12.8 MP, and I found the quality not perfect, but much better than simply cropping. A shot of a batter from the outfield fence for example requires an 800mm, 1200 if you want to fill the frame on FF.

Then I acquired a 7D. Some time later I sold my Version II extenders and bought version III for both 1.4X and 2.0X Extender, which were better.

Now that I have a 7Mark II, and my 5D is a Mark IV, I find I don't use the extenders much. Cropping works seemingly just as well. I have owned both a 1DX mark 1 & II over time, but sold them.

I have toyed with the idea of purchasing an 800mm IS lens, but when I look at image quality comparisons on The digital Picture (dot com) the quality seems similar to my lens and the 2X extender V III.

Just looking for a discussion here.