You can use an RF extender with an EF adapter, but you will need to use a modified adapter. The Canon-made EF-EOS R adapter is not compatible with RF extenders. However, there are a few third-party adapters that have been modified to work with RF extenders.
Here are a few of the modified adapters that are compatible with RF extenders:
- Commlite EF-RF adapter
- Viltrox EF-RF adapter
- K&F Concept EF-RF adapter
When using an RF extender with an EF adapter, you will need to keep in mind the following:
- The maximum aperture of the lens will be reduced by the amount of the extender. For example, if you use a 2x extender with a lens that has an f/2.8 maximum aperture, the maximum aperture will be reduced to f/5.6.
- The image quality may be slightly degraded, especially at the edges of the frame.
- The autofocus performance may be slower, especially with telephoto lenses.
If you are considering using an RF extender with an EF adapter, I recommend doing some research to see if the benefits outweigh the drawbacks for your particular needs.
I hope this helps!