Years ago, I bought a Canon 12mm extension tube for one purpose: to stack my Canon Extender EF 2x III teleconverter and my Canon Extender EF 1.4x III teleconverter with my Canon EF 600mm F4L IS II lens on my Canon EOS-1D X Mark II camera. This setup achieved a stunning 1680mm focal length.
I used it with Canon EOS-1D X Mark II only a few times. The focus was very slow, and image quality, of course, was not the sharpest. After some processing, some of the images were usable.
Image captured with Canon EOS 1D-X Mark II + 1.4x TC III + Canon 12mm extension tube + 2x TC III + 600mm F4L IS II lens.
I now have a Canon EOS R3 as my primary camera. I decided to test stacking teleconverters with this camera system. I used the same EF mark III teleconverters. Since it was a cloudy day, the ISO was relatively high. Because of that, I opted to try to record the video instead. The autofocus was slow, but once it found the subject, it could keep it in focus.
I connected my lens, teleconverters, extenders, converters, and camera in the following order - R3 + RF-EF adapter + EF 1.4x III TC + 12mm extension tube + EF 2x III TC + 600mm F4L IS II. The camera still reported a 1200mm focal length and f8 aperture.
This is the video I captured:
Disclosure: As an Amazon Associate, I earn from qualifying purchases.
Check out my gear on Amazon: https://amzn.to/47lOk0P
Want to get started with trail cameras? Check out my Stan Store for a guide - https://stan.store/kallepihelgas
Want to get your own trail camera? Check out the affordable starter kit (Amazon US) https://amzn.to/3QStjWq