While this is my first post to this forum, I have been reading and learning from it for a while... so I thought it was time to try giving something back .
Yesterday for the first time, when I tried to use my Canon 70D with a kenko DGX (blue dot version) 2x extender and the Canon 400 5.6L, the camera basically froze, refused to turn off or write pictures to card. The only way to resume it was to remove the battery.
The same problem (or some variations of it) kept reappearing every time I try that combo. The teleconverter works fine with other lenses, and the lens works fine with other teleconverters. At first I could not figure out what was wrong... also because I successfully used that combo before!
Then I remembered I read something similar here regarding the 5D mark III:
I thus realized that the reason for the new behavior was that I recently tried to AF microadjustment that lens (luckily, no need for that, or maybe needed just -1 in some situations).
I then switched off the AF microadjustements, and everything worked flawlessly again. By the way, the new Dual pixel AF works quite nicely with that combo, allowing AF at 800mm/F11 (sometimes it can hunt a bit, but it is fast enough to take pics of a perched bird)! Too bad you can't microadjust...
Hope this helps!
The blue dot 1.4 also refuses to work on my 60d and 70d with the 400 f5.6, but works perfectly with 1dmk2n. It also worked (and autofocused) on the 60d with a 100-400 L.
I have MFA on the 70d but the 60d does not have MFA so must be a different issue. Going to try your fix and see if i can get it working on the 70d.
Pity if it doesnt as i bought the converter specifically for this lens and body. Lucky for me its working on the 1D and images look great..
I had this kenko TC, 60D and 400mm F5.6L and it's does AF on 60D with 400mm F5.6L, BUT the centre point is extremely unreliable... you need a very contrasty target for the centre point AF to lock on. The strange thing is the outer AF points work better. Same issue with 70D.
By the way, it work perfectly with 6D... as long as you don't activate the MFA.
Have a nice day.