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

[AlanM04]

Hi,

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.

Thanks

[wq9nsc]

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.

Rodger

[AlanM04]

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.

[deebatman316]

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 

[AlanM04]

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.