i have the 100-400 and 300 4.0 which with 1.4x is just noticebly better than the 1-4 at 400. i would suspect that the 400 5.6 is just noticebly better than the 300 4.0 +1.4.[a href=\"index.php?act=findpost&pid=67592\"][{POST_SNAPBACK}][/a]
My 300 f/4L IS is not as sharp as my 100-400 at 400, wide open. The 300 needs to be stopped down a bit, at which point it is a little sharper than the zoom. The 300 with a 1.4x underperforms the zoom at 400 in every way. I get more subject detail with the zoom with a 1.4x, and 2x. with them stacked, very little extra subject detail is captured. I intend to get another Kenko 1.4x, because I like to keep one on the camera for birds, and stacking to 2.8x is usually a waste, or impractical because of lighting. Two 1.4xes would be better, in terms of returns. I only like to use the 2.8x stack when the subject is in direct sunlight. The extra magnification in the viewfinder makes manual focusing more precise. In the dark, it makes it worse.
With the 2.8x stack, I get a lot of misses due to tremors, misfocus due to rocking or subject movement, but when I get a hit, I get a lot more subject detail than I would without the TCs, or with just a 1.4x.
I believe that people are too quick to judge TC usefulness based on the 100% pixel view, which I think is really kind of irrelevant. For the type of photography one is doing when they need more reach and use a TC, the subject is what matters, and 2x more detail in 4x as many pixels is a gain, even if the pixels themselves lose contrast. Cropping is not a viable option IMO, with the level of artifacts present, when light is low. Many people say that an upsampled crop is better than using a TC, and that may be true for certain lenses, and may be true with AF, but it is not true in my experience. My most detailed bird shots are almost always the ones taken with the TCs.
It takes a lot of skill to use TCs properly, and judge when to use them or not, especially when combined with slow lenses. They are not "plug and play".
