Using 1.4 extender with EOS R6 and RF adapter

[anthonyrjwright]

I am contemplating buying the Canon R6 Mk III pus the RF adaptor so that I can continue to use my 100-400 and 70-210 EF lenses.  I also have an EF 1.4 extender.   Will this extender/zoom combination work with the R6 and adapter?

[SignifDigits]

You can use the Canon RF to EF adapter (there are two, one has a control ring and one does not) on the R6 Mark III and full-frame EF lenses and/or EF Extenders.  You may (likely will if you're like most of us) want to move to native RF lenses at some point, but the EF lenses will work.  There are AF concerns that you should be aware of regarding manual focus ring adjustment in AF modes on many EF lenses.  From what I can find, both of yours should not be focused by you manually when the camera is set to drive the AF.  Doing so could damage your lens.

[shadowsports]

Greetings

I would be cautious.  I would absolutely not try to adapt the EF 70-210 (either variant) 1987 f4 or 1990 f3.5~5.6 to an R6 mkIII.  Will it fit = Yes.  What will the performance be = anyone's guess?  Performance might depend on what's acceptable to the user.  It's from a different era.  

The EF 100-400 mkI was released in 1998.  It's known to have compatibility issues with teleconverters.  The mkII was released in 2014. John had one.  @jrhoffman75       I think he tested it with a 1.4x TC too.  I think we should ask his opinion.  He has an R6mkIII now too.

https://community.usa.canon.com/t5/Gear-Guide/EF-100-400mm-f-4-5-5-6L-IS-II-USM-Extender-EF-2X-III-L...

[jrhoffman75]

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@shadowsports wrote:

Greetings

I would be cautious.  I would absolutely not try to adapt the EF 70-210 (either variant) 1987 f4 or 1990 f3.5~5.6 to an R6 mkIII.  Will it fit = Yes.  What will the performance be = anyone's guess?  Performance might depend on what's acceptable to the user.  It's from a different era.  

The EF 100-400 mkI was released in 1998.  It's known to have compatibility issues with teleconverters.  The mkII was released in 2014. John had one.  @jrhoffman75       I think he tested it with a 1.4x TC too.  I think we should ask his opinion.  He has an R6mkIII now too.

https://community.usa.canon.com/t5/Gear-Guide/EF-100-400mm-f-4-5-5-6L-IS-II-USM-Extender-EF-2X-III-L...

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I did have the Mk II version. Used it with my 1D X Mark III. Very pleased with it. I also had the 1.4X extender. 

I did some informal comparisons between using the extender and cropping bare lens to same image size. I couldn’t detect a difference in images. Faster AF and no light loss. 

Never used it on my R6. Sold my EF lenses and converted to RF. 

While AF capabilities and camera performance may be impacted by various EF lens limitations I have never seen anyone reporting that optical performance was degraded. 

Since your use case might be different and since you already have the EF optics you can make your own evaluations. Only out the cost of the adapter which you could ultimately sell if you don’t like the results. 

[johnrmoyer]

If you have version II of the 100-400, then I think that the lens with extender works very well on EOS R5 which has a higher resolution sensor than the R6. At long distances there will be some distortion caused by differences in air density between camera and subject. If there are insects or dust in the air, out of focus objects over a long distance will accumulate to soften the image and change the colors.

I posted earlier about this at: https://community.usa.canon.com/t5/Gear-Guide/EF-100-400mm-f-4-5-5-6L-IS-II-USM-Extender-EF-2X-III-L... 

I use Canon DPP software which knows about the extenders so that "digital lens optimizer" does very well at removing the disadvantages in image quality caused by using the extenders. Rawtherapee free software in "capture sharpening" does a Richardson/Lucy deconvolution to remove small aperture diffraction blur.

A more recent example: https://www.rsok.com/~jrm/2025Nov25_Salt_Plains_NWR/2025nov25_eagle_IMG_5100c.html

Bald Eagle (Haliaeetus leucocephalus) at Salt Plains National Wildlife Refuge in Alfalfa County, Oklahoma, United States on November 25, 2025

Camera Model Name	Canon EOS R5
Lens Model	EF100-400mm f/4.5-5.6L IS II USM +2x III
Focal Length	800 mm
Exposure Time	1/400
ISO	500
F Number	13.0
Camera Temperature	34 C
Measured EV	13.75
Measured EV 2	30
Focus Distance Upper	164.68 m
Focus Distance Lower	81.91 m

100% crop:

100% crop

[SignifDigits]

Thanks to my colleagues for adding more detailed information.  I thought about delivering just a one-word answer "Yes" to the OP's question, but thought I should add the caution about the focus ring.   I'm thankful others are addressing the very important unasked question " Will the I be pleased with the results?"  I didn't try to address that important concern as I could not and am glad others have the experience here to do so.  I believe consensus would be that switching to the full-frame mirrorless RF system will almost certainly lead to wanting to move from EF to RF lenses due to some potential frustrations with EF lenses and desiring the advantages the RF lenses have to offer.  That is certainly my personal experience, though sometimes the price point of some of the really excellent lenses in the huge and excellent discard pile of used EF lenses has proven too hard to resist.  I purchased the EF 100mm 2.8 L IS and 180mm  L 1:3.5 for macro work at very reasonable prices with no regrets.  I also purchased the EF 70-200 mm L IS III over the RF Z version to save $1,000 and still get the internal zoom.  I think I might have also given up some speed on the AF, especially via the R6MarkIII.  Too early to say about my regrets there.  I may have to save up for that upgrade.   So, for the OP, it really is good to go in with your eyes wide open about "working" versus "will I really be happy that I made this change only to find learn that I really need to also plan on replace my EF lenses at some point" is a really good thing for this group to point out - I apologize for my oversight in that regard.  But, as seems to always be there case here, the outstanding group steps in and covers oversights!

[ebiggs1]

My one word answer is no don’t do it. Of course it depends on what you are good with. And how you intend to use or view your work. The eagle crop is soft but who does 100% crops as a normal final product. It’s actually pretty good if it was the 100-400mm and tel-con.

[johnrmoyer]

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@ebiggs1 wrote:

My one word answer is no don’t do it. Of course it depends on what you are good with. And how you intend to use or view your work. The eagle crop is soft but who does 100% crops as a normal final product. It’s actually pretty good if it was the 100-400mm and tel-con.

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It was in fact made using the 2x extender which degrades the image more than the 1.4x. It was not possible to get closer to the eagles so this photo would not have been possible hand held from a car window without the extender.

Here is another version of the same photo that has not been changed by posting to the forum: 

https://1-john-moyer.pixels.com/featured/eagle-5100-john-moyer.html 

The metadata in the image at https://www.rsok.com/~jrm/2025Nov25_Salt_Plains_NWR/2025nov25_eagle_IMG_5100c.html is intact for lensmodel 

exiftool -s -G0:2 -lensmodel /home/jrm/public_html/2025Nov25_Salt_Plains_NWR/2025nov25_eagle_IMG_5100c.jpg 
[EXIF:Image]    LensModel                       : EF100-400mm f/4.5-5.6L IS II USM +2x III