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EF 300mm f/4 L USM Aperture Won't Hold



Strange issue has recently started when using this lens. Happens on both my 7D bodies. 

Manual mode with auto ISO f4.0 various fast shutter speeds (outdoor sports)

Will take a few hundred shots at f4.0 but then the aperture will jump to 5.6 and I cannot manually dial it back to 4.0. I have to disconnect the lens and reconnect it. The camera will then stay at f4.0 for a while but will eventually flick between f5.6 and f4.0 randomly. Never seen anything like it before and wondered if anybody else has experienced this behaviour.




The only setting I can think of that might cause something like this is a safety shift setting that causes the aperture to close down if the camera thinks it is over-exposing at the selected shutter speed if the lowest available ISO still creates over-exposure.

I am not familiar with the 7D body setup but I have these settings disabled on my 1DX series bodies so that it won't automatically do something contrary to my chosen exposure settings.


EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video

Thanks for your reply. I can't immediately see any settings that fit your description but I've tried my 400 f4 and 70-200 f4 and both of these are fine. I'm thinking there's an issue of some sort with the lens.


This sounds like a communication problem. It sounds like there is an invisible 1.4x teleconverter mounted to the camera. Hence why your camera is reporting the aperture as F/5.6 because you lose 1 stop of light. Does the camera report that a 1.4x teleconverter is mounted. In another forum something similar was happening to the OP in that forum. They had a limited aperture of F/5.6-36. When the max aperture of the lens was F/2.8 so something wasn't right. The correct aperture range was from F/2.8-22 but the OP couldn't set a wider aperture. Lens with similar problem 


Current Gear: EOS 5D Mark IV, EF Trinity, EF 50mm F/1.8 STM, EF 85mm F/1.8 USM, 470EX-AI & 600EX II-RT

Retired Gear: EOS 40D & Sigma 17-50mm F/2.8 EX DC OS HSM

Thanks for your reply. Yes, I'm also starting to think there's an issue with the lens and this 'phantom' teleconverter certainly makes sense. I bought the lens used a couple of years ago from MPB (UK) so I think I'll email them and see what they suggest.

I'll report back as and when I get an explanation.