I tested the 400 f4.5 + 2x TC on a couple of birds out my front window. So, I am shooting Z9 + 800mm f9 in lowish light due to rain and overcast conditions and thus relatively highish ISO. I used as low a shutter speed as I could to try to keep ISO's as low as I dared to go with a subject that may possible move and at this long focal length, any movement of subject is *noticeable*.
The 2x TC does take the edge off sharpness but it is still very useable. These birds are at about 6-8 mts from the camera so, no cropping required. 800mm for these photos is too close as I could not get the whole bird in the frame with a little room around it for "breathig space" and "correct" composition. Ideally, I would say that for a standard sized bird like these and a pigeon, you would need to be about 10-12 mts away to get a correct frame filling photo with a little room around it. For a larger chicken sized bird maybe 15-20mts away. I have not had the opportunity to shoot at longer distances with the 2x TC yet.
In this situation, I would shoot the lens bare and crop if required (see last photo). However, if I was at say 10-20mts away, I would use the 1.4x TC and crop if required. If I were at say 20-30mts away I would possibly use the 2x TC and crop if required or possible. Using a TC is never cut and dry as it depends on distance and ISO used etc and thus you need to weigh up the situation as to whether use a lens bare, with a 1.4x TC, or 2x TC, try to get closer or walk away from the shot.
Most of the seasoned long lens users here will already be well aware of all this but, for those that are considering this set up, 800mm is quite long and excellent long lens craft is required. 800mm and f9 wide open is no point and shoot option as it requires a steady hand, low shutter speeds to keep ISO down and a target that stays still. Any movement of camera, lens or more importantly, subject matter, is magnified even if you have VR and IBIS available. Due to the low shutter speeds involved here, I took many photos to get some decently sharp shots mostly due to subject movement.
No noise reduction is applied, but my usual post processing via Capture One Pro, adding a little saturation, and where required, maybe highlight/shadow, WB and levels. Open in Photoshop reduce for web size and sharpen a little.
Z9 + 400 f4.5 + 2x TC, 1/160s f/9.0 at 800.0mm iso1400
Z9 + 400 f4.5 + 2x TC, 1/160s f/9.0 at 800.0mm iso1800
Just to give context, this shot was taken the day before with the lens bare at about a 1/2 a meter longer distance than the last photo above.
Z9 + 400 f4.5 bare, 1/250s f/4.5 at 400.0mm iso1000 as seen in the post above
The 400 f4.5 is tempting but it is an extravagance that I don't need and would only get if I was silly enough to sell my 400 f2.8. The 100-400 is excellent anyway and is a handy zoom range.
