
Did a little duck hunting with my D7100 and Nikkor 200-500 today
On the walk out into the marsh I had the Kenko 1.4 TC attached - on the way back it was off. Not an exact comparison but it was a learning experience. While have a 35mm equivalent reach over 1050mm (that's what shows up in the metadata) that sounds marvelous. There is some compromise. AF is significantly slower to lock - but does - I'm getting better at it. But I.Q. suffers too. Is that extra reach worth it? Good question.
Given that distance to bird and lighting would be different in these two shots it is not a fair comparison but overall when it comes to BIF's especially the T.C. is debatable advantage.