Cool! This is great news! I almost bought the 1.4x III extender earlier this week because I was afraid the 2x extender would not work with some lenses on my 1DX. Now the choice is simple! I will get a 2x extender.
Using the 1.4x will yield better optical quality than the 2x, and also with an f/4 lens and the 1.4x you have 21 cross-type AF points and 40 more single-line points, whereas with an f/4 lens and the 2x you have 1 cross type and 4 single-line points.
So, budget permitting, having both makes sense to me.
Good point Neuro. Thanks for the suggestion. I have always been afraid to get extender because of what it could do to the image quality. I forgot about the number of AF point available...Maybe I will start with the 1.4 anyway afterall. If I dont like the resulting quality then for sure I would not like the effect on the 2x extender!
I can see the difference on the TDP ISO chart section but in real life situation, do you find the 1.4x III extender really degrade your image?
First image is the bare 600/4 II. Second image is the 600/4 II with the 1.4xIII. They were actually shot 1 week apart, and with full disclosure, the bare lens takes a hit for being shot on a cloudy late afternoon, while the sun was shining a week later when I returned to the same spot. So, the bare lens image is f/4 ISO 6400, while the 1.4xIII is f/5.6 ISO 3200.
The last image is a pair of 100% crops, bare lens on the left, 1.4xIII on the right.
You be the judge...