A camera body will show F00 and manual focus (MF) when it cannot communicate with a lens, which could be because there is no lens attached, because the lens is fully manual, or because there's a problem with the lens itself.
I went out birding over the weekend, mounted my usual combination of 600/4 II, 1.4xIII and the vanilla EF-EOS R Mount Adapter to my R1 and headed out. When I turned on the camera, it showed F00 and MF. I thought, "Oh, sh!t," and swapped on the 2xIII. That worked...but then it also changed to F00 and MF. So I mounted just the bare lens...and that was perfectly fine so I spent the day shooting like that.
When I got home, I cleaned all the contacts and tested various combinations again, including changing out the vanilla mount adapter for the drop-in version and my 3rd party adapter modified for an RF extender to fit behind it.
The upshot was this, and the situation is seemingly stable (through many trials over a couple of days, no changes). Happens on both the R1 and the R8, and with all three of the mount adapters.
- Bare 600/4 II – functions normally
- 600/4 II + 1.4xIII – F00 and MF
- 600/4 II + 2xIII – varies between:
- Normal function
- Showing an aperture value that can be adjusted, but still only MF
- F00 and MF
- The variation is caused by physical manipulation – twisting the lens in the mount or moving the lens, e.g., lifting it from pointing down into shooting position, affects the functionality
- 600/4 II + RF 1.4x (with Commlite adapter) – functions normally
The fact that I see this behavior with two bodies and three adapters suggests the problem is the TCs or the lens.
I haven't used the 2xIII TC quite some time, not since Comet C:2023 A3 Tsuchinshan-ATLAS in October 2024. So maybe the 2xIII failed sometime over the past 18 months (while sitting mostly undisturbed in a Pelican case with a dehumidifier unit), and the 1.4xIII failed sometime in the past 3.5 weeks since I last used it.
If it's the lens and not the TCs, the problem is specific to use with EF TCs and yet the problem exhibits different symptoms with the two TCs, and I don't know why that would be the case. Admittedly, I would prefer it to not be the lens, because the service life for the 600/4 II ended last year, so sending it to Canon is not an option. I could replace it with the RF 600/4, but I suspect we'll see a version of that lens with the 1.4x TC and hopefully fairly soon. I would love to have the latter and would not love buying twice.
One other idea occurred to me, literally as I was typing this post. I started to write that I don't have any other TC-compatible EF lenses with which to test the extenders...and realized that while that is true, I do have three other EF
mount lenses that are physically compatible with the extenders, but don't report them to the camera body (TS-E 17, TS-E 24 and MP-E 65). So I tried the 1.4xIII and the 2xIII with the TS-E 24, and I found that the 1.4xIII shows F00, and the 2xIII shows an aperture value that can be adjusted.
It seems rather unlikely that both TCs independently failed, but I am thinking that's exactly what has happened. Sort of a Sherlock Holmes, "
When you have eliminated the impossible, whatever remains, however improbable, must be the truth," sort of thing.
From a practical standpoint, assuming that I'm correct and both extenders have failed, I see no need to replace the 2xIII. One option would be to replace the 1.4xIII ($480 new or $280 used), but I can also just use the RF 1.4x behind my modified Commlite mount adapter, and just remember that I'm doing that since it's not reported in the EXIF. The downside to the latter is that I would not benefit from the DxO lens profile for the combo (though I could batch edit the EXIF and I suspect that DxO would use the 600 II + EF 1.4xIII profile, it still won't be the right profile).
Thoughts, suggestions, and sharing prior experience welcome!
Edit: the RF 1.4x itself does show up in the EXIF, but the exposure information doesn't reflect it, remaining 600mm f/4.
. And yes, I'm suppose to know that it's all about the regulation (aka non-protein codding DNA).
thx in advance!
). I hope it is not the lens. Can you borrow (or rent) another EF 1.4 TC to verify that it is not the lens?
All of my lenses are much fresher that way. And movies. And my own age, to be honest. 
