I'm using 5D MKII body and 100-400 f/4.5-5.6 lens. I want to use a 1.4 converter; will the Kenko converter work and autofocus properly?
I have used this combination before, and it really depends. Most of the Kenko 1.4 TCs will allow you to AF without any kind of tape tricks, however only when there is enough light. If you have particularly bright lighting conditions, then AF will work...albeit very slowly. In any other situation, AF will be spotty at best, if it works at all. In lower light situations, AF will usually just hunt back and forth, quite slowly, without ever actually locking on to anything.
You need to make sure you attempt focus on high contrast areas, preferably obvious contrast (not just microcontrast). If you try to focus with the Kenko+100-400 on an area that is largely uniform or similar in tone, then AF will usually just hunt.
I would also like to point out that, even assuming you DO manage to get AF working well enough to lock consistently, the 100-400 just doesn't have the base IQ necessary. Slapping on a TC will greatly exacerbate corner and midframe IQ issues, and the smaller maximum aperture immediately limits your diffraction-limited center-frame resolution.
Overall, it is really not a worthwhile option to use the Kenko 1.4x with the 100-400. It is really not a worthwhile option to use any TC with the 100-400...it just doesn't really perform well or produce keeper IQ. You are much better off saving your money for a better lens (i.e. 300mm f/2.8 can handily take a 2x TC III and still produce better IQ than the 100-400 on its very best day!)
If you need a larger subject in the frame...the best bet with the 100-400 is to figure out how to get closer. When it comes to wildlife and birds, some camo pants and shirt will go MUCH farther than a TC towards getting you more frame-filling shots, without any IQ loss (rather, with an IQ improvement, assuming your subjects don't flee.)
jrista's answer sums it up rather well in terms of whether it is advisable or not to use any teleconverter/extender with the EF 100-400mm f/4-5.6 L IS. There is a lot of inaccurate information in several of the other answers and comments that should probably be cleared up.
My experience is primarily based on using a Kenko 2.0X Teleplus PRO 300 with a Canon EF 70-200mm f/2.8 L IS II mounted on a Canon 7D. I have tried other combinations of lenses and bodies just for the sake of experimentation. Here is what I have learned.
- Most older Canon cameras rated to focus at f/5.6 or wider, unlike newer bodies that may be firmware limited to turn off AF above f/5.6, will still attempt to focus when an f/8 combination is mounted. Sometimes they even succeed. For example, an EF 24-105mm f/4 L IS + Kenko 2X PRO 300 + Canon 5DII will Auto Focus in bright conditions. The combination focuses noticeably slow, but does work. The IQ from that combination is also very good in conditions where the primary source of light is behind the camera. The same combination mounted on a 7D (APS-C camera, thus smaller mirror and narrower focus array) will hunt for focus but not generally succeed unless it is pointed at an area of very bright/dark contrast such as a bare light bulb, and then only certain focus points will acquire focus lock. Note that this is only the case with third party converters. The Canon extenders appear to use firmware to disable AF with f/8 combinations and AF won't even attempt to find focus with an f/8 combination mounted.
- My Kenko convertor and every other one I've seen for Canon EF mount includes the same number of pins as my Canon EF lenses do. Since the Canon EF mount has no mechanical connections, without the pins there would be no way to stop down the aperture or use Auto Focus. My convertor correctly reports the aperture to the camera body. If I have an f/4 lens + 2X Kenko mounted the camera sees it as an f/8 lens. Apparently when the EOS system was developed back in the 1980s, the EF mount was designed to allow the provision for a lens with a manually set aperture ring on the lens. Thus the lens reports two different numbers to the camera: one that is the current aperture setting and another that is the maximum aperture of the lens. Third party converters and even lenses can fool the camera by reporting the lens as an f/5.6 lens set at f/6.3 (In the case of several third party zoom lenses that are f/6.3 when at maximum focal length) or even f/8.
- Although I've never used the EF 100-400mm f/4-5.6 L IS, based on my experience with Canon bodies and lenses when combined with Kenko teleconverters, I would expect the 5DII/2X Kenko/100-400mm combination to attempt to focus and mange to succeed in brighter conditions. Due to the basic IQ of that lens when used bare, I would not expect the results from such a combination to be worth the trouble.
Test image using Canon 5DII + Kenko 2.0X Teleplus PRO 300 + EF 24-105mm f/4 L IS. Taken about 2-3 hours before sunset under a clear sky. One Shot AF. ISO 400, 210mm, f/8, 1/640 sec. AWB. Contrast and sharpening moderately boosted in post.
I had a similar configuration (with a Canon extender). It worked in manual focus by default. With the "tape the contact" trick focus did work, but was only usable in bright light, anytime close to dusk, even with my poor technique I would switch back to manual focus.
Your best bet is to check the specifications of your intended teleconverter and your camera. A 1.4x teleconverter will cost you 1 stop of aperture speed, thus, if your camera is able to auto-focus at f/8, then it should still work as long as the teleconverter handles the necessary adjustments to preserve auto-focus capability (no all telecovnerters do, since it requires updating the aperture information sent to the camera.) Only a few cameras support AF at f/8 though, so it probably will be a problem.
Focusing will also be slower, though exactly how much slower depends on how good your AF sensor handles the f/8 focusing. You will also likely have fewer AF points available to you since typically only a few of them are setup to work at slower apertures.