topic Which extender for my R5II and 100-400 L USM II in EOS DSLR & Mirrorless Cameras

Which extender for my R5II and 100-400 L USM II

Chickadee12 — Sun, 24 May 2026 22:00:02 GMT

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

Re: Which extender for my R5II and 100-400 L USM II

deebatman316 — Sun, 24 May 2026 23:07:59 GMT

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.

Re: Which extender for my R5II and 100-400 L USM II

Waddizzle — Mon, 25 May 2026 12:08:11 GMT

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.

Re: Which extender for my R5II and 100-400 L USM II

ebiggs1 — Mon, 25 May 2026 12:57:25 GMT

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.

Re: Which extender for my R5II and 100-400 L USM II

johnrmoyer — Mon, 25 May 2026 13:10:16 GMT

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

Re: Which extender for my R5II and 100-400 L USM II

Waddizzle — Mon, 25 May 2026 15:48:58 GMT

“ 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.

Re: Which extender for my R5II and 100-400 L USM II

deebatman316 — Mon, 25 May 2026 15:55:37 GMT

@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.

Re: Which extender for my R5II and 100-400 L USM II

Waddizzle — Mon, 25 May 2026 17:33:57 GMT

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