https://community.usa.canon.com/t5/EF-RF-Lenses/Autofocus/m-p/138425#M11041 <P>"Works" is a relative word here. &nbsp;It probably will work but it will not work well.</P><P>&nbsp;</P><P>Have you considered, you already have a tele advantage fo 640mm with that lens. &nbsp;Maybe the only place where a EF, or full frame, lens has an advantage is on a crop body. &nbsp;640mm is going to be a challenge to deal with. &nbsp;Nearly 900mm is really&nbsp;tough. Strong tripod. &nbsp;Everything has to be right for the shots to be worthwhile.</P> Tue, 14 Apr 2015 13:35:17 GMT ebiggs1 2015-04-14T13:35:17Z https://community.usa.canon.com/t5/EF-RF-Lenses/Autofocus/m-p/138325#M11039 <P>I intend to use the Canon 100-400 Mk11 with a 1.4 tele converter, will the AF work ok? (using a Canon 7D Mk11)</P> Mon, 13 Apr 2015 15:37:39 GMT https://community.usa.canon.com/t5/EF-RF-Lenses/Autofocus/m-p/138325#M11039 Chevi 2015-04-13T15:37:39Z https://community.usa.canon.com/t5/EF-RF-Lenses/Autofocus/m-p/138337#M11040 <P>It does work but most likely center AF point only and it will be a bit slower.</P> Mon, 13 Apr 2015 16:35:35 GMT https://community.usa.canon.com/t5/EF-RF-Lenses/Autofocus/m-p/138337#M11040 cicopo 2015-04-13T16:35:35Z https://community.usa.canon.com/t5/EF-RF-Lenses/Autofocus/m-p/138425#M11041 <P>"Works" is a relative word here. &nbsp;It probably will work but it will not work well.</P><P>&nbsp;</P><P>Have you considered, you already have a tele advantage fo 640mm with that lens. &nbsp;Maybe the only place where a EF, or full frame, lens has an advantage is on a crop body. &nbsp;640mm is going to be a challenge to deal with. &nbsp;Nearly 900mm is really&nbsp;tough. Strong tripod. &nbsp;Everything has to be right for the shots to be worthwhile.</P> Tue, 14 Apr 2015 13:35:17 GMT https://community.usa.canon.com/t5/EF-RF-Lenses/Autofocus/m-p/138425#M11041 ebiggs1 2015-04-14T13:35:17Z