Originally Posted by
DanK
I've done this with the following equipment:
Canon 100-400 II
Canon 1.4 x II
7D (old version)
5DIII
I set the lens at 350 mm on the crop (560mm FF equivalent) and 400mm + 1.4x on the full frame. I shot a high resolution chart at a short distance, perhaps 15 feet.
The crop without the converter beat the FF with the converter. The image with the converter wasn't as sharp, but the biggest difference I noticed was more chromatic aberration with the converter. I don't have a Mark III TC to try, but my guess is that it would be better. I don't think I kept the results, but the differences were enough to convince me to take the crop rather than the FF plus converter when I know I will want reach.
Unfortunately, my old 7D won't focus with the 100-400 at 400 plus a converter.