I just recently got the TC 14III to use with my D500 and my Nikon 200-500. I'm in the process of getting used to it, so my views will undoubtedly change as I adjust. My early impression is that it does a terrific job if you're a mediocre photographer like me. It requires good light and is a bit slow to focus (but not very slow), but holds detail well and allows for a real increasse in reach. The exif in this photo says that the eagles were 56 meters away, which seems about right to me. I have cropped by a factor of two in this image, but it would have been a lot more without the TC. It's true that the fine detail in the feathers is lacking, but at that distance I am very pleased with the detail that it got. It was a grey day and the sky was blown out, so I pushed the background warmth to my visual taste.
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Sorry -- I thought the image would expand to full size. Here's a crop that will hopefully show at 100%:
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Double-click on the image of the crop and it looks pretty much like 100% to me.
FWIW