"The Canon 70-300 L is officially NOT compatible with Canon extenders. This is the only Canon L zoom lens with a widest focal length of 70mm or longer that is not compatible with Canon extenders (as of review time). I've tried mounting Canon extenders to the 70-300 L - and they do not fit. The rear 70-300 L lens element physically hits the The high-performance Canon Extender EF1.4X III and Extender EF2X III increase the focal length of the attached EF lens by 1.4X and 2X respectively. aSafety Precautions • Do not look at the sun or a bright light source through the lens with the extender attached, or with a camera with the extender and lens attached. List of lenses that are compatible with the EF 1.4x II and EF 2x II extenders. Solution. The following lenses are compatible with the EF 1.4x II and EF 2x II extenders: The Canon 1.4x lens extender extends the focal length of a lens by 40%, making (for example) a 200mm lens function as a 280mm lens. the version III 2x extender cannot be combined with a 1.4x "Depending upon how exacting your tastes, " More to the point is, depending upon how exacting your requirements. In other words, how are you to use the photos? IME, no 2x tel-on is worth using for any reason. However, I find the 1.4 tel-con works reasonably well on the ef 70-200mm f2.8L zooms. My advice is don't do it. Actually don't use any 2x on any lens. The 1.4x tel-con is useable on a very few Canon lenses but not the 2x. The ability that well it fits is not a good reason to use it. Of course it is all about what you want and your requirements are. If you can settle for mediocre phot The EF 400mm f5.6 is supported. According to the manual, it will AF, but if trouble is encountered, then manual focus should be employed. Ability will depend on shooting conditions and the lens. Normadel is correct minimum aperture is f11 at 800mm. The reduction in light passing through the lens, is similar to stopping down the aperture, or reducing the size of the lens aperture. A 1.4x extender changes the effective focal length of a 100mm f/4 lens into a 140mm f/5.6 lens. Notice how both the focal length and f/stop have been multiplied by 1.4. Greetings, This information might help. The EF 400mm f5.6 is supported According to the manual, it will AF, but if trouble is encountered, then manual focus should be employed. Ability will depend on shooting conditions and the lens. Normadel is correct minimum aperture is f11 at 800mm extender- EOS 90D & EF 100-400mm - Auto Focus doesn't work with 2x extender in EOS DSLR & Mirrorless Cameras 11-25-2023; Mount Adapter EF-EOS R question about compatibility with Extender EF 1.4x III in Camera Accessories 11-14-2023; Interesting EF 400 f2.8 IS II issue - EOS-1D X Mark III rendered unresponsive in EF & RF Lenses 11-12-2023 qrfb.