Canon 2x III Extender

[jcc285]

I have just purchased a EF Extender 2x III and am attaching this to my 5D Mk IV EOS Camera with a EF 100 - 400 1:4.5-5.6 L IS USM lens. The AF switches off when the extender is in place and 'Manual Focus' shows in the display on top of the camera. Removing the Extender and re-attaching the lens returns the camera to AF AI Servo. What am I doing wrong or what have I not done i.e. do I need to tell the camera that the extender is there? Any help gratefully received.

[diverhank]

---

@jcc285 wrote:

I have just purchased a EF Extender 2x III and am attaching this to my 5D Mk IV EOS Camera with a EF 100 - 400 1:4.5-5.6 L IS USM lens. The AF switches off when the extender is in place and 'Manual Focus' shows in the display on top of the camera. Removing the Extender and re-attaching the lens returns the camera to AF AI Servo. What am I doing wrong or what have I not done i.e. do I need to tell the camera that the extender is there? Any help gratefully received.

---

This is normal for this lens.  When you mount a 2X extender, you lose 2 f/stops so with this lens with an f/5.6, it becomes an f/11 lens and your camera can only focus with f/8 or less.  You should return and get a 1.4X which will maintain focus with this camera/lens combo.

[jcc285]

Thank you, I hadn't understood that the camera would only focus at
f/8

[jrhoffman75]

Actually, very few Canon cameras (only the high end ones) will AF with an f/8 lens. Most max out at f/5.6. 

[TCampbell]

You can use a 1.4x extender with the 100-400mm lens ... which takes it to f/8.  That's still allows for working auto-focus.  

The 2x extender was meant for f/2.8 lenses (which takes them to f/5.6) but since the 5D IV can focus up to f/8, you can get away with using it on an f/4 lens (which takes it to f/8).