Which extender for my EOS R5 Mark II and EF 100-400mm f/4.5-5.6L IS II USM?

[Chickadee12]

I want to get a 1.4x extender to give me a little more reach.  (I know others that do this and it works well with very little difference in quality.  I think the 2x extender might limit me too much in lower light situations.)  I of course use the R to EF adapter.  Should I get the R 1.4x or the EF 1.4x III extender.  I have read that most use the EF,  but in the future I may change to a R lens (depending on what Canon comes out with) so wonder if I put the extender next to the body and the adapter next to the lens will that work? 

Anne

[deebatman316]

Hi @Chickadee12 RF Teleconverters designed NOT to mount to the EF-RF Mount adapter. Your only choice is the EF Teleconverter. I would highly recommend using the Mark III Teleconverter if you have to use a Teleconverter. I person recommend using a lens that has the correct focal length. As Teleconverters reduce the amount of light entering the camera. Also the AF motor slows down too. Not to mention that the lens may take an optical performance hit when a teleconverter is used.

[Waddizzle]

Demetrius is correct.  The RF teleconverters are not designed to work with the RF-EF mount adapters. But ai see no reason why an EF III mount adapter would not work. I don’t recommend using the original EF nor the EF II mount adapters. Both were released prior to the lens.  The adapter firmware cannot be updated. 

[ebiggs1]

Bad idea all around. Don't do it. No matter what anybody tells you a tel-can always extracts a price for the minimal gain in focal length. In photography there is no free lunch you always give to get.

[johnrmoyer · Accepted solution]

I have the EOS R5 and not the version 2. I frequently use EF100-400mm f/4.5-5.6L IS II USM +1.4x III or EF100-400mm f/4.5-5.6L IS II USM +2x III. Since I already had the EF100-400mm f/4.5-5.6L IS II USM, this cost me less than buying an 800mm lens. The DPAF handles the focus issues that the small aperture caused on my EOS 80D. 

I have read that some have modified an EF-RF adapter so that the RF extender would fit, but that does not seem the best solution to me.

Yellow-rumped Warbler (Setophaga coronata) in Norman, Oklahoma, United States on January 8, 2026

https://www.rsok.com/~jrm/2026Jan31_birds_and_cats/2026jan08_warbler_IMG_5507c.html 

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	1/500
ISO	1250
F Number	13.0
Camera Temperature	33 C
Measured EV	12.75
Measured EV 2	24.5
Focus Distance Upper	10.56 m
Focus Distance Lower	9.07 m

Tufted Titmouse (Baeolophus bicolor) in Norman, Oklahoma, United States on April 3, 2026

https://www.rsok.com/~jrm/2026Apr30_birds_and_cats/IMG_5771c_2026apr03_titmouse.html  

Camera Model Name	Canon EOS R5
Lens Model	EF100-400mm f/4.5-5.6L IS II USM +1.4x III
Focal Length	560 mm
Exposure Time	1/1250
ISO	2500
F Number	9.0
Camera Temperature	30 C
Measured EV	11.88
Measured EV 2	21.5
Focus Distance Upper	18.79 m
Focus Distance Lower	15.15 m

All of the edits in Canon DPP software

AngleAdj	0.5
WorkColorSpace	sRGB
WhiteBalanceAdj	Daylight
PictureStyle	Shot Settings
UnsharpMaskStrength	1.7
UnsharpMaskFineness	4
UnsharpMaskThreshold	5
ToneCurveOriginal	Yes
LuminanceNoiseReduction	5
ChrominanceNoiseReduction	5
DLOSetting	40
CropRotatedOriginalWidth	8239
CropRotatedOriginalHeight	5535
CropX	1508
CropY	956
CropWidth	4500
CropHeight	3000
CropRotation	0
CropAngle	0.5
CropOriginalWidth	8192
CropOriginalHeight	5464

[Waddizzle]

“ The DPAF handles the focus issues that the small aperture caused on my EOS 80D. “

The DPAF sensors are a huge step up in performance. I think the R5 used the same sensor as the 5D4.  The Mark II versions of the R6 & R6 were another performance improvement. 

[deebatman316]

@Waddizzle actually the EOS 5D Mark IV & EOS R (Original R) use the same sensor. The EOS 6D Mark II & EOS RP also use the same sensor. The EOS R5 (original version) was the first R series camera to have a completely new sensor. The EOS R6 (original version) & EOS-1D X Mark III use the same sensor.

[Waddizzle]

I stand corrected. Don’t know how I forgot about the R and Rp. 

[ebiggs1]

Bottom line is, all that the person is OK with and how much post editing you have to apply that's important. Not my cup of tea.

[Chickadee12]

Thanks everyone!  I just returned from the "Biggest week in birding" at Magee Marsh, Ohio and I talked with a lot of photographers who find the 1.4x works great.  Yes, I know I will lose a little light and gain some noise, but with Lightroom's new upgrade to noise reduction/raw details I am not too worried.  I recently took pictures of a Rufous-lored Kingfisher in dark dreary lighting - I almost fell of my chair when I saw what Lightroom restored, and it gave better results than Topaz in less time and an additional .tiff file.  Raw details fixed the colors back to what the bird looked like, instead of a dark brown lore. Thanks for the tip on the speed issue. I will remove the extender when speed & light is critical (hummingbirds, tapaculos etc).  Being prepared for where a bird might land or anticipating its next move is much more important than the blip in time it take to focus this setup.