The Canon EF 70-300mm DO USM manual says it will not work with the EF 2X II extender. Will it work (particularly the IS) with the newer EF 2X III extender?
No, it will most likely not be fully compatible with that lens. Check the compatibility chart.
Do you understand why it will not work with some cameras? BTW, which camera are you using?
Using an extender is not a free lunch. The price you pay is in the form of less light entering the camera, which effectively raises the f/stop of the lens. While the extender multiplies the focal length of a lens by 2x, it will also do the same to the effective f/stop. Because of the reduction in light, AF systems tend to slow down slightly with an extender.
The 2x extender will raise your f/stop minimum range by 2 full stops. Your f/4.5-5.6 range will become f/9-11.2. The firmware in most cameras disable the AF when the minimum f/stop is at f/8, or narrower, because their AF systems cannot get sufficient light to focus accurately and consistently.
The 1.4x extenders will raise your f/stop minimum range by only 1 full stop. Your f/4.5-5.6 lens will effectively become f/5.6-8, but the AF system may not work properly at all zoom focal lengths. If it works at all wiith a 1.4x, the AF will likely work at the shorter focal lengths, and not at all at the longer ones.
Overall, you should expect the AF system to not be as fast compared to using the lens without an extender.
The Canon extenders only work with certain lenses because of a physical reason. The extenders have a front projection that goes into the rear of the lens. Most lenses have the rear element very close to the mount.
One exception to to the compatibility list is the 70-300L zoom. As long as the lens is kept at 300mm the extender will fit. If the lens is zoomed shorter the lens can bump into the extender.
12/05/2023: New firmware updates are available.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.