I have a very old 400mm / f5.6 APO lens that I used with my EOS 5 film camera. I had tried it with 300D, 5d MK II, and my latest 5D MK IV, but could not get it to work. However, I could occasionally take a couple of shots and then I was getting non-communication messages! Then I had a "flash": I set the aperture at 5.6 and switched to manual mode and manual focus. As long as the aperture was 5.6 the lens would work without any problem. Then I tried Av mode and set the aperture at 5.6 and it worked. I switched to auto-focus and this also worked.
Conclusion: As long as you keep the aperture at the lens max. aperture the lens works. Obviously only on manual and Av modes. Tv is out of the question.
Try the trick on old Sigmas that you may have.
Conclusion: As long as you keep the aperture at the lens max. aperture the lens works. Obviously only on manual and Av modes. Tv is out of the question.
Try the trick on old Sigmas that you may have.
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