Tested this combo under a solid overcast gloomy day. Tired of waiting for the SUN !
Tripod mounted, Delay Timer, AF at F5.6 without the extender and of course AF with the 2X extender at F5.6, F8.0, F11.0
Simply moved the tripod back to frame the test subject close to the 1st capture without the TC at F5.6
None of the below captures have been cropped and no major PP performed other then adjusting the exposure in DPP a small amount due to the fact that when using a 2X extender under certain ambient lighting conditions it will sometimes either over-expose or even under-expose your shots.
Also did 100% crops and they are below in another post.
AF was very responsive with the 2X and i also performed these tests with Live View, X10, delay timer, manual focus, & even at MFD and the results were very good, but decided to post the below since AF performed flawless and thats what most folks want when purchasing an extender.
The minimal AF speed reduction could possibly be the result of the less light fall-off % due to just the 4 element internal design of the MC4 versus the 7 elements in the MC7`s. Who Knows ?
On my 24" monitor F8.0 appears to be the target to use with a crop body sensor.
Included below are also some random captures taken handheld with estimated distances to subject included.
400mm F5.6, ISO200, ss 1/500
400mm F8.0, ISO200, ss 1/250
400mm F11.0, ISO200, ss 1/125