I recently bought a used Canon 200-400 f4 lens and have used it for several hours of sports shooting with no problems. Last night while shooting it stopped being able to autofocus and none of the buttons on the camera would work. I tried switching bodies thinking it was my camera, but the same thing was happening. After lots of experimenting, I could get it to work right if I disconnected the lens from the camera while the camera was powered on and then reconnected. It would only work correctly for a short time though. Anyone have ideas on what might be wrong or fixes I could try?
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The lens should be returned if its not working. Since its a used lens.
When this happens does the camera still report lens aperture. If the camera displays F/00 or F/-- that could mean the flex cable in the lens has failed. Or the lens contacts may need cleaning. But this rarely fixes the problem. Also what camera are you using this lens with. It is also not an easy fix. Canon can replace it if the lens is still supported by them. But get a quote first before repairing if its still supported by Canon.
Are you hitting the AF Stop button on the lens on accident. Also what camera are you using this lens on. EF 200-400mm lens manual PDF
I was thinking that it is also possible that one of the array of AF stop buttons on the lens is jammed if he isn't touching it. The array of AF buttons around my L telephoto primes is a feature I really like but that might be a part of the lens most subject to damage if it was heavily used in bad weather/sea side conditions.
Does the camera report MF when this happens. The AF stop button will cause the lens to act this way on both cameras. Even if it is intended or not. That also doesn't mean that the AF stop button is functioning correctly either. In the camera's custom functions menu. You can turn off the AF Stop button to see if its causing the problem.
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