This is one of the first images taken with the Canon extender 2X on my Canon 70-200 f4.
It was a surprise to see how it comes out.
Considering that this images is 640 ISO I must say that what can do the difference is to use RAW instead of JPEG.
RAW is able to minimize rumours and keeps quality of your images very high.
Esposizione 0,003 sec (1/350)
Lente 330 mm
The image above is taken with the same lens ith the 2X on, but here the ISO value is higher, here is 1600.
The image comes out from a RAW but despite the ISO here the quality is still quite good.
Esposizione 0,008 sec (1/125)
Lente 400 mm