Sat 05-Jan-13 02:07 PM | edited Sat 05-Jan-13 02:19 PM by Leonard62
Good post, Brian. You covered the areas that I can't and that's the Sigma line of super zooms and the super telephotos from Nikon. But this question comes up often and for someone on a budget and using Nikon gear here's what I can show.
First I don't think the AF 80-400mm lens is bad like some people think.
Here's a comparison of three rigs that most people mention. Shot with the D3X at ISO 400.
Left photo . AFS 70-200mm VR II with Nikon TC-20E III. 400mm f5.6 Center photo. AF 80-400mm VR .. 400mm f5.6 Right photo. AFS 300mm f4 with Kenko Pro 300 1.4X . 420 mm f5.6
All three photos were shot at a distance greater than 100 feet, are 100% crops and were all taken within 5 minutes of each other. Other than sizing there was no post processing. The photo with the AF 80-400mm is clearly sharper.
I have run this comparison a few times in the past and even with the D3200's much higher resolution, the results are always the same. It is very hard to beat the AF 80-400mm lens with other lenses and a TC. The one exception I have found is the AFS 300mm f2.8 VR with the Nikon TC-14E II.
edit: I also used the AFS 300mm f4 with the Nikon TC-14E II but found the shot using the Kenko TC to be slightly sharper so that's what I used in my comparison.