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The Magnification ratio of a 100-400 F4.5 to F6.3 lens is 4 to1 add a two times TC and those F4.5-F6.3 figures F9 to F12.6 may not really sing as sweet as one would hope unless its blindingly bright sunlight. ….
The Z 100-400 is f5.6 at the long end, so with the Z 2x TC it is f9 to f11. Still a 2 stop penalty in light/aperture, as you note.
I have the two Z TCs which I initially bought to use with the Z 70-200 before the Z 100-400 came out. The Z 1.4x TC is better, but the Z 2x TC is still quite good and usable on this lens.
As I noted above, I have used the Z 1.4x TC on the Z 100-400 and think it is quite good optically if losing a stop of light/aperture is acceptable.
In his recent talk on Nikon long lenses, Brad Hill noted that the Z 100-400 was quite sharp through 350 mm and was less sharp at 400 mm (but still useable and definitely not soft). Brad noted that the Z 2x TC worked well with the Z 100-400 up to 350 mm (for a total combination of 700 mm with the TC). There was another thread on the forum talking about Brad’s presentation and there are links to it in that thread and on Brad’s website.
I was out in my kayak two days ago using the Z9 with the Z 100-400 and the Z 2x TC. I was photographing a couple of loon families and their chicks. I decided on this outing to limit myself to 700 mm or less, given Brad’s comments. Still going through my shots. F11 can be challenging if the light is low, as it was when the clouds came over and the loons were in shade and darker water (along shore, based on reflections of the pine forest). Given the wind and waves, the kayak was drifting and rolling a bit. Same for the loons, in addition to swimming slowly. So I could not drop my shutter speed too low. Normally I would prefer 1/2000 or 1/2500 here, but dropped it to 1/1600 and 1/1250 and shot in bursts. Moved it back up to 1/2000 whenever the sun came out.
Pretty high ISOs when the sun was shaded and the water dark. Topaz DeNoise helped. But in this case, at 700 mm, I’d be better off with the 500 mm PF + 1.4x TCIII, as that would get me to 700 mm but is an f8 combination rather than f11.
On the other hand, the loons can be closer at times and zoom flexibility is often highly useful. I could take a 500 mm PF and TC on the Z9 and the Z 100-400 on a Z7II, but that makes my kayak cockpit a bit crowded and AF on the Z7II is not as good as the Z9.
I liked a number of the shots, especially when there was more light. Even at 700 mm, I ended up cropping them significantly. There’s only so close you can get without scaring them off or disturbing them. Some loon families are more skittish and others less. (And you can get closer in a kayak than any other water vehicle I have tried.) I converted my HE* raw files in ACR with no sharpening or noise reduction (other than color noise reduction) and then used Topaz DeNoise and Topaz Sharpen in Photoshop. May have sharpened them more than I would have done with the 500 mm PF and 1.4x TCIII.
There are certainly trade offs here. And I will do more testing for the kind of shooting I do. But my initial reaction is that the Z 100-400 plus Z 2x TC is worthwhile when I need a longer focal length and zoom flexibility (and don’t want to have 2 bodies with me).
