first use of my 2x extender

It's all about right tool for the job. With any camera/lens/TC you can get lucky occasionally and have a dead-on focused shot. But as noted by the OP, choose the wrong combo and the hit rate goes way down.
I was using the 70-200 IS II with 2.0 TC III on my 5D mk3. Mostly aviation and birds. By itself the lens is fantastic but it does t play well with the 2x TC III. At least not for critical situations. And it's slow focusing on the 5D mk3 AND my 1Dx. Wouldn't even bother on a 5D mk2. Add low light or low contrast to the mix...well..your problems seem normal for that combo.

The TC affects your aperture along with focal length. The 70-200 f2.8 becomes a 140-400 f5.6. This is why your settings changed. I'm assuming you were in one of the auto modes if aperture, shutter and ISO changed.

Canon's AF systems work on contrast. If the focus points can't see enough contrast they will hunt. This is true even on the more advanced AF systems. Difference is the more AF points, the greater chance of getting a hit. The AF system also looks at the closest object when multiple points hit. So there is skill involved in placing the points exactly where you want. A while back there was a video from Canon that explained this very well. And after playing with the AF system to test what the rep was saying it changed the way I thought about the AF system.

If you have to get to 400mm look at the f5.6 which isn't terribly expensive. Great IQ and focuses really fast with good light. Could also look at the 100-400 but the ergonomics of the push/pull take a bit of getting used to. Also, its aperture varies with zoom which drives me nuts. Either however should increase the "hit" rate. Sell the 2x.

And then practice a lot! Even with the best gear there will be a lot of wasted shots until the skill is there to back the gear up.

I usually rent the 2x III extender when i'm shooting something as it's still a bit pricey here (€500), and i have shot sports, concerts, birds and much more with it. It works fine as long as you are able to calculate the lag it introduces when you autofocus and then learn to compensate for it when you shoot your subject. This calculation and compensation game will help you nail your shots, it did it for me.

Recently i covered a whole indoor concert from 30 meters distance in a poorly lit indoor stage arena with the 2x III extender on my 300 2,8 lens mounted on the 7D and also on the 5D2. Few were the shots which were out of focus and bad, and that's at f/5,6. If you know what you are doing it's fully capable and useful but you need to know your gear inside out to get the most out of it.

/ Magnus

Magnus3D wrote in post #15609044
Recently i covered a whole indoor concert from 30 meters distance in a poorly lit indoor stage arena with the 2x III extender on my 300 2,8 lens mounted on the 7D and also on the 5D2. Few were the shots which were out of focus and bad, and that's at f/5,6. If you know what you are doing it's fully capable and useful but you need to know your gear inside out to get the most out of it.

/ Magnus

To be fair, a concert generally doesn't include objects that are moving as fast as kids sledding or a bird in flight or an airplane zipping by at 130 mph...or jet at a few hundred mph. So there's plenty of time to calculate lag.

Oh.. Just remembered..my avatar was done with the 1Dx, 70-200 II and 2x TC. Compared to what I'm getting now with the 500 f4 II and 300 f2.8 II it's night and day. And the 500 doesn't like the 2x TC, loves the 1.4 though. The 300 loves both TC's

And yes..i know there's a wee bit of a price gap

I guess the point I want to make is that even the best gear and knowledge/skill won't matter if the conditions are bad. Had a paid shoot for an aircraft company that I was lucky to get 12 of about 100 pics in focus due to terrible light. (Cloudy and just past sunset) but it was the only chance to get the plane in flight before it left for China!

You could not be more wrong, try artists jumping around on a poorly lit stage. That is a challenge even with fast glass and a camera with good AF. The movements are just as random and unpredictable as those a child produces.

/ Magnus

The concerts I've shot must have had better lighting. I keep the shutter speed high enough to freeze action and rarely miss a focus. Plus depending on the artist, they won't let us do much closer than a half body framing. So we're full body or wide enough that movement doesn't affect focus that much. I had one bass player for a metal band who was a complete lunatic for an entire show and I couldn't get a decent shot of him but other than that, the movements have been pretty predictable.

coeng wrote in post #15606653
So basically you're telling me I bought it for the wrong reason? I was hoping to use if for extending my range when shooting sporting activities that my kids engage in.

Keep using it. I own the mk3 version.

A few people mocked it. It does a great job.

Enjoy using it

There is a learning curve with most new gear. You can get good results with that combo. Don't give up on it. 100% keepers, never. With practice you can get some photos that you will treasure.

The 2x III works allot better on a 300 f/2.8 IS than on a 70-200 f/2.8 IS II. The TC's work best on the primes. Even then you need decent light and practice to make the most of the 2x. The 1.4x works good on the 70-200 though.

You've found out you can't beat native focal length. TC's have a place, but a 2x is asking a lot of your equip in challenging situations. That's not to say you can't get it to work it just requires more skill/luck to get what you want.
Centre point 5D2 AF is supposed to be reliable though, are you shooting outer AF points ? ? ?

I think you'd be much better off with the 1.4x

Even on the best lens, the 2X is a lot too ask from a lens. Your doubling all it's designed specs, (FL and F/Stop) which is asking a lot.

butterfly2937 wrote in post #15647112
The 2x III works allot better on a 300 f/2.8 IS than on a 70-200 f/2.8 IS II. The TC's work best on the primes. Even then you need decent light and practice to make the most of the 2x. The 1.4x works good on the 70-200 though.

I have not (yet) tried my 2 x Mk3 extender on my 70-200 F2.8, however I regularly use it om my 300 F2.8 L IS Mk1 and am very impressed. I have compared images against my 600 F4 L IS Mk1 (now sold to finance longer lens) and was surprised at how close the quality was. Naturally the 600 was better but the 300 + extender at half the price and half the weight was far from shabby in reasonable light. The 2 x really excels on this lens.

If you are shooting at the 200 mm end of the lens switch your lens to mode 2.
I have the 70-200 2.8 is mk1 and 2x mk2, shots dont start getting sharp untill F11. You need good light..

I dont recall if the OP stated his shutter speeds and distances he is shooting from.
However, remember that the 70-200 2.8 that becomes a 140-400 5.6 is going to require a long lens handholding technique.

Many people take some time to learn to hold the lens more still (400mm is big for most people) and many people are used to shooting 1/200th or slower and that isnt gonna happen easily for someone who just stepped up to 400mm.

give it time. I have friends that get very good results with that combo.

I use my 2x MkIII with my Canon 70-200 f2.8L IS MkII quite regularly on my 5D MkIII and get some great results. This combo works very well and takes full advantage of the 5D MkIII's fantastic AF system but it does take a hit in the AF speed and IQ. Once you get used to it you can get the best out of it and get some great results, but bear in mind the AF system of the 5D MkII is fairly poor compared to the MkIII. I found the MkII to be fantastic for most types of shot but action shots are a challenge with it at the best of times.