I have actually used the 2x III extender with the 28-300. You have to manual focus and you MUST be dilegent to keep the lens longer than 50mm or you will impact the rear element into the extender. I help protect against this by using the tripod mount ring and clamp it on at the 70mm line.
For me the purpose of the extender is to get me out in the 400-600 range so I'm never less than the 200 anyway or I'd just leave the lens stock. I found the 70 line to be a good balance point for me. I just slip a business card in the ring to help it
I found this out as I had the extender for another lens and figured I'd fiddle with it, not wanting to buy another. Because I actually liked the 28-300 so much I got ride of the other lens and kept these two since I found a work around for the incompatibility. I just can't afford another L series lens, and the flexibility of this one is just to great to part with.
I am certain this should not be a practice for many or even most, however I did find I could get it to work and got the extra range out of the lens. Anyhow, just thought I'd mention that.
Perhaps it might help someone. Do be clear though, you will have to manual focus.
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