Things to keep in mind when reading various review.
1. There is no such a thing as free lunch. Anyone who claims fast autofocus with this combo is smoking something. Canon white papers claims a 75% reduction in autofocus speed. It will also degrade image quality, no matter what lens you put it on.
2. As for Image quality, acceptability, and degradation, there are several factors in play. Of course where you end depends on where you start. used on an optically superb lens, after some image degradation, you can still end up with good image quality. with mediocre lens, you end up with bad, and with a bad lens, you end up with mush. The other factor in acceptability is the image density of the sensor. The image quality will be a lot more acceptable on a 5d2 or 40D for example than the 7D. the higher pixel density of 7D is far more demanding on lens resolution. furthermore, the cropped sensor of same pixel count, greatly magnifies Chromatic aberration.
3. In addition to loss of image quality secondary to use of extenders, going from say 200 to 400, requires great improvement in long lens technique. any movement is greatly magnified. for someone to properly assess the functioning of the lens, esp when used for things such as birds in flight, great technique is required.
4. there are obvious things that one must know. in addition to double the focal length, you loose 2 stops of light with the converter, so a 2.8 lens becomes a 5.6 lens. if you are starting with a lens with max aperture smaller than 2.8, you will loose autofocus on all current bodies, except for the 1D/1DS class. The coming 1DX has also dropped support for lenses slower than f 5.6.
Edit april 2013.
As many are aware, Canon updated firmware in 1dx to now support f8 focusing with central focus point, and has promised similar upgrade with canon 5dIII. so autofocus is still possible with the TC mounted on a f4 lens on those bodies.
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