jdnan wrote in post #18690199
Here is an example of my 300 w/ the 1.4x, III. I'm actually getting better results than I've ever achieved when attempting to use the extender in the past. I haven't changed anything so I don't really know why other than I know exactly where the hummingbird is going and I can limit the focus range of the lens to 2.4-6m. When I've attempted to use the extender in the past, it has been with large birds like owls, etc. at much longer distances. Of course there is a noticeable fall off in sharpness from straight out of the 300 w/ no extender. These images are of course heavily cropped with some basic adjustments (highlights, shadows, clarity) in Capture One pro.
Evaluating how well the setup works at ISO 12800 is a bit of a strain for getting a baseline, even on a 1Dx II. It looks like you're actually recording good detail but losing it to such a high ISO. Your ISO 5000 shot looks like a much better result, at that small of a photo anyway.
Shots of a static subject, from a tripod, manually focused at 10x in Live View should give you your "best case" scenario as a 'before' step to gauge any other changes from. Then, a couple of the same shot in AF One Shot to see if it's hitting at the same spot or whether it's front/back focusing.
If you're getting consistent front or back focus, MFA the combo on that body; then retest.
Once you're getting acceptable results there, it's time to work on exposure settings and/or technique.