First, the original Canon EF 400mm "DO" simply did not work well with teleconverters... 1.4X or 2X. The 400mm "DO" II purportedly works a lot better with them, but I don't use it and can't say from personal experience.

My advice is to avoid 2X teleconverters, when possible. For the best image quality use no more than 1.4X.

Teleconverters always work best on prime lenses and less well on zooms... though the IQ produced varies greatly depending upon the specific lens and TC combo.

Canon 70-200mm f/2.8L IS USM "II" works very well with Canon 1.4X III and pretty well with 2X III.

Canon 100-400mm L IS USM "II" works well with Canon 1.4X III.

Canon cameras that will autofocus with an f/8 lens & TC combo...

Full frame:
- 1DX Mark II (all 61 points)
- 5D Mark IV (all 61 points)
- 1DX (center AF point out of 61 AF points, four surrounding points "assist", requires later firmware version)
- 5D Mark III, 5DS, 5DS-R (center point of 61 AF points, four surrounding points "assist", 5DIII requires later firmware)

APS-C Crop sensor:
- 80D (27 of it's 45 AF points)
- 7D Mark II (one center point of it's 65 AF points).

Older full frame 1DS models and APS-H 1D models also have some f/8 capability (typically center point only, out of 45 total AF points).

All other Canon cameras are "f/5.6 limited", meaning that when they detect a lens/TC combo that makes for an effective aperture smaller than that, they are designed to turn off AF and force you to focus manually.

Keep in mind that 100-400mm with 2X will be an effective f/11 at the long end (anything longer than approx. 300m in the zoom range). Not only will the AF turn off, but the viewfinder will be heavily dimmed down as well, making manual focusing difficult. Focus Confirmation also will not work, when AF is disabled due to the lens/TC combo. So that's not available to help you manually focus, either.

Some things you can do....

#1. If using a full frame camera, consider supplementing it with an APS-C model, particularly for long lens use. This will be sort of like getting a "free 1.6X teleconverter". By "free" I mean that unlike an actual TC, there's no "light loss". On my APS-C 7D Mark IIs, my 100-400mm II lens "acts like a 160-640mm" would on my full frame 5D model. That makes for versatile, sharp lens and a 20MP image. Cropping a 5D III's 22MP FF image down to the same amount would leave you with only about 9MP... cropping a 5DIV's 30MP down the same would leave you about 12MP image. Not bad, in either case.... but not as good as 20MP! BTW, 7DII's sensor density is almost identical to the 50MP 5Ds-models', except the ISO range of 5Ds/5Ds-R is 100-6400, expandable to 50 and 12800. 7DII's is 100-16000, expandable to 25600. About a year newer than 7DII, 80D offers 20% more resolution with a 24MP image on APS-C, as well as more f/8 capable AF points (27 versus 1 in 7DII). 80D has slower continuous shooting rate (7 frames per second versus 10 fps with 7DII) and it's not as tough or well-sealed a camera, and maybe not quite as fast-handling... But it has same ISO range and many think offers the best APS-C sensor from Canon to date (a wee bit higher usable ISO and a little greater dynamic range).

#2. If using a TC/lens combo that exceeds the camera's AF rating, it's possible to tape up a couple of the contacts on the TC so the camera doesn't know it's installed and will still try to AF. One minor drawback is that EXIF metadata in images will be incorrect... it will only show the lens focal length. This also might effect distance info if using ETTL II flash. And you also should expect AF performance to decrease significantly... more AF hunting and failure to lock on. How good or bad AF works will depend a lot upon ambient lighting conditions, subject contrast, etc. If interested in trying this, Google for more info online. I know I've seen web sites that show which contacts to tape up, but don't have a link handy.

#3. Live View might be helpful if the viewfinder is too dimmed-down for manual focusing. Enabling Exposure Simulation in Live View can brighten the image displayed on the LCD screen. Also, you can magnify the Live View image to check manual focus.

Personally I use the Canon 1.4X II and 2X II teleconverters.

The 1.4X II I use on Canon 135/2L, occasionally on EF 70-200/2.8L IS USM (rarely, there's a lot of loss of IQ on the original f/2.8 version), frequently on EF 300mm f/4L IS USM, EF 300mm f/2.8L IS USM and 500mm f/4L IS USM. I have not yet had occasion to try it on EF 100-400mm IS USM "II" which I added to my kit last year (and use a lot on 7DII). I also haven't tried it on EF 70-200mm f4L IS USM zoom.

I use the 2X II almost exclusively on EF 300mm f/2.8L IS USM, occasionally on my 500mm... never on any zoom. Image quality takes a much greater hit with a 2X.... where there might be 5% loss with a 1.4X.... there's probably more like 15% or more loss to a 2X. But there's huge variation in the degree of loss of image quality, depending upon the exact lens/TC combination.


Canon EF 500mm f/4L IS USM with EF 1.4X II, on Canon 30D (APS-C, 8MP)

The Canon "III" teleconverters are also said to be quite good. Personally I did not see need to upgrade from "II"... In part that's because I simply don't need to use wither with zooms. I have prime lens and TC combos that work very well and make that unnecessary.

I've also heard a lot of good things about the Kenko 1.4X, which are a whole lot less expensive. Their cheaper "MC-4" is quite sharp in the center, some think even sharper than the Canon, but less so in the corners and at the edges. Often some corner softness doesn't matter very much for wildlife photography. But if it does using that TC on an APS-C camera will mostly just use the "sweet spot" of the image area. If you will be using the TC on full frame and want better edge-to-edge and corner-to-corner sharpness, the Kenko "Pro 300" model might be preferable. It's slightly more expensive (though still about 1/3 the price of the Canon "III"!) There are Kenko 2X MC-7 and Pro 300, too... though I don't know how they compare.

If you'd like to compare for yourself, Bryan Carnathian's the-digital-picture.com has both sample images and magnified test shots done with many lenses and lens/TC combinations. You can compare side-by-side there, too... at different apertures, various zoom focal length settings, with and without TC, and on different cameras. For example, http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=972&Camera=979&Sample=0&FLI=4&API=1&LensComp=972&CameraComp=979&SampleComp=0&FLIComp=5&APIComp=2 shows the 100-400mm II wide open performance without any TC at 400mm (left) and with 1.4X III at 560mm (right). Switch the right hand display to 800mm to see how it performs with 2X III. You also can choose smaller apertures to see how it improves and compares. At that link the test shots are all done with 5Ds-R.... just about the most demanding camera available. You may be able to switch both over to another camera model that's more like what you use. (Note: I compared the 400mm "DO" original with 2X II against the 400mm "DO II" with 2X III and the newer combo certainly looks a lot better to me!)

If you are looking for examples of the 400 DOII with extenders, as others have suggested, you can check out "Regis". Remember though he is using a 5DSR, which many would say is not well suited for wildlife, but he would argue it works fine. If you would like a few examples of the lenses IQ on a 1DX or 5DIV, I have several posts. Here is an example: http://www.uglyhedgehog.com/t-447876-1.html. Hope the link works.

The 100-400 II I understand is an excellent lens, very versatile for relatively close wildlife subjects. But with a 2x on it, I would think it's getting a little too slow and you would be limited to only bright subjects unless you start pushing the ISO and begin to affect IQ. Additionally it's my understanding very few bodies (if any) will AF with that combo. I don't think any Canon bodies have full AF function at f/8 except the 5DIV and 1DXII and the 100-400 with a 2x will be greater than f/8 (apparently as someone noted, a software upgrade to the 5DIII allows it to AF at f/8.0). From that perspective, I speculate the 2X converters are designed to work with long primes at f/4 and not the zooms (except for f/2.8 lenses). So, if you are serious about using a 2X converter, I think you need to stick with primes (400 DOII, 500, or 600). Now we're talking big $. You can take the plunge (remember the big Canon primes hold their value quite well) or you could save a bunch of money and and go with a high resolution body (5DIV or 5DSR) with a 100-400 and a 1.4 converter and crop.

If cash is not an issue and you are serious, the best of all worlds in my opinion: 400 DOII and/or a 600mm IS II with converters on a 5DIV or 1DXII. But it is a big investment.

Good luck!

Here are my thoughts ......smaller and lighter is better. That makes the 400 DO THE choice. ( for a prime). Zoom with your feet is easy to say - but much harder in the field. Zooms are far more versatile than primes. I have shot the Canon 300 2.8 W/2X II - and no noticeable degradation ( and I am fussy) - but I do use good techniques and support. Your long lens techniques and management will outweigh the native optical IQ of the lenses regarding final IQ. If you are on a high MP body, cropping if necessary and using pixel enlargement software if necessary trumps using an TC - IMO.

First, I replied to that note and not to you directly because he mentioned potential IQ issues and I wanted to additionally remind him about the AF issue. Often when people post they refer to the OP so everyone reading it will know who we are referring to. Additionally, I respond to a lot of threads and I often am not familiar with most OPs handles and the specific spelling of them, so it's usually easier and quicker to respond to them as the OP. No offense was intended and none should be taken.

May not be important to you, but all sharpness/IQ issues aside, the 500mm and the DO are an f4, vs the zoom. At the technical end, for me, that puts the primes into a faster lens group. With out checking it might be an A group. That means that ALL of the cross focus points in the camera are working, not just some of them and the multi-cross points are activated as well. So the primes will lock focus faster, especially in low light.
To a pro, these are VERY important factors and to me, even more important than IQ because the IQ is gonna be good enough! Always consider that no matter what the IQ is, if the camera CAN'T lock focus then IQ is a useless attribute. That's the ONLY reason that the 400 2.8 even exist!!! Just saying.
SS