The 7D AF system won't work if the combined lens plus 2X aperture is above f5.6. So unless you are using a f2.8 or faster lens, the 2X will put your 7D out of AF range. A lens with a maximum aperture of f4 combined with a 1.4X will autofocus with your 7D because the one stop loss from the 1.4X results in a f5.6 combo. The same f4 plus 2X results in a 2 stop loss of aperture putting the 7D out of its AF capable range.
Even with the sharpest primes I own (EF 200 f2, EF 300 and 400 f2.8), results with the 2X converter are most underwhelming. With the sharp primes, the image looks OK until you compare and see how much IQ degradation occurs compared to the bare lens. With lesser glass, the results will be even less impressive.
The 1.4X does work quite well with many lenses and as long as the bare lens is f4 or faster, it will AF with your 7D.
The 1 series will AF with aperture combinations as slow as f8 but even with my 1DX III the AF is noticeably slower and less sure with this combination (i.e. using an EF 800 f5.6 with the 1.4X which results in f8 combined and it will AF but nothing like the lightning quick results with the bare lens).
I had to hang around the house waiting for a service tech, so I decided to run a little test.
I set up my 1D X Mark III on a tripod and mounted a test image on the garage door.
With the EF100-400 f/4-5.6L II mounted with the 2X VIII telextender I framed the shot, trying to pretty much fill the frame.
Then I mounted the 1.4x and finally no extender (1X).
In Canon DPP I turned off all sharpening and performed the Digital Lens Optimizer process.
Once that completed I switched to the Fine Detail Picture Style.
Once completed I cropped the 1.4X and 1X images to match the 2X as close as I could. Then I exported them as 16bit TIFF files. File sizes were 114MB, 54MB and 27MB (2X, 1.4X and 1X).
Then I did a Windows screen capture which produced JPEGs.
I'm curious what you folks think. From my observation and this limited test my thoughts are 1. the 2X image doesn't seem bad and 2. if one doesn't have a telextender cropping the basic lens, even with just a 20MP body, gives pretty good results.
For reference, here is the full 1X image and a 100 percent zoom.
So I have also now come across this problem with the 2X Converter. I have Canon EOS 6D Mark II, Canon EF 100-400mm f/4.5-5.6 L IS II USM Lens, and have both the Extender EF 1.4X III and the Extender EF 2X III. I have had no problems at all with the AF using any of the above combinations up until today.
I was shooting with just the 100-400 lens, and wanted closer so I put on the 2X Extender, for the first time ever, the auto focus will not work, it has been working fine since January when I bought the extender, and now no matter what I try, it stays in manual focus.
Anyone have any ideas? I think maybe I put the Extender on the camera by accident without the lens attached, and then before doing anything, fixed that, and attached the extender to the lens and then to the camera. The AF has not worked since.
Any solutions? Maybe reset the camera to factory default maybe? The 1.4 Extender works fine, the lens alone works fine, as does my 24-105 lens. Just the 2X III will no longer AF
Are you sure it ever focused? The 6D Mark II has an f/8 aperture limit for AF, same as my 1D X Mark III, which does not AF with the 2X TC. The 100-400 f/4.5-5.6 would have an f/9 equivalent at 100mm with the 2X TC.
Not sure what to say about all of that, but the auto focus has been working with the 2X III for 3 months. Oddly enough, I just put the 2X III back on the 100-400 and the auto focus is working again. No question it is working, I just put the camera completely out of focus with the manual ring, and a couple of 1/2 presses of the shutter and bang, focused. I did understand when I bought the 2X III that it would "slow down" the auto focus due to losing the 2 stops, so it would not be good for panning or following a moving target, but it does work for distant still targets. Anyway, it is working again and I have no explanation why it stopped or started working again.
I do not see any way to attach an image here, but it works within the entire range of the 100-400 so 200-800. I have many sharp images at 800 mm equivalent using the auto focus, the CN Tower here in Toronto is 5 miles away, and lots of lunar shots at night, etc. Could it be because of the 2X III newer version or software updates or something? All of this stuff is relatively new, the 6D and 100-400 was last June so 9 months old, and I got the 2XIII in January so 2 1/2 months of using it with the 100-400. All firmware updates were done on everything I have.
The last image I took of the tower on 2022-03-20 the camera details in the properties is f/11, 1/80 sec, ISO-200, 800 mm, and that image is very sharp for 5 miles away.
As the originator of this thread, I gave up with the 7D and bought an R with which the 2x AF works a treat!
I'm really impressed with this camera which was the only mirrorless I could afford. I'd love to try the R5 to see how much better it performs and would be interested to hear if anyone out there has made this comparison.