topic Re: Extender EF 2x III compatible with EF 100-300 f/5.6L on EOS R? in Camera Accessories

Extender EF 2x III compatible with EF 100-300 f/5.6L on EOS R?

rickmp001 — Mon, 13 Jul 2026 04:23:06 GMT

Hello,
Is the Canon Extender EF 2X III compatible at all with the EF 100-300 F5.6L on a Canon EOS R body with Mount Adapter EF EOS-R? This is not on the compatibility list, but I was wondering if this combination would work, even if only in manual focus mode. Have been a Canon shooter since 1975 with an F-1 body (non-N variant). Thank you...

Re: Extender EF 2x III compatible with EF 100-300 f/5.6L on EOS R?

Waddizzle — Mon, 13 Jul 2026 11:00:03 GMT

How well does it work without the extender? That’s your answer. The lens released in 1987. It’s in the Canon archive.

https://global.canon/en/c-museum/product/ef266.html

It was designed for film cameras. Digital lenses use coatings on the internal elements to minimize light reflecting off the sensor back up into the lens body. Thievrelections create haze in the image, similar to what you might see shooting through a window.

As for the teleconverters, you might be better using a crop sensor body.

Re: Extender EF 2x III compatible with EF 100-300 f/5.6L on EOS R?

shadowsports — Mon, 13 Jul 2026 12:24:03 GMT

Greetings ,

I would not recommend using this lens on a mirrorless body. With or without a TC. While adaptation may be possible, a lens this old is not going to be up to the communication standards or the performance required and offered by a mirrorless body. Even in manual mode you'd be setting yourself up for disappointment if it worked.

If you want to buy a budget 600 mm lens for use on a R series body I would recommend a.Sigma or Tamron 150-600, a RF 600mm f11, or an RF 200-800 for about $2k. if money is no object, the RF 600mm f4 is $15k,

Don't go the Frankenstein route and hope for the best because you will be disappointed.

Re: Extender EF 2x III compatible with EF 100-300 f/5.6L on EOS R?

johnrmoyer — Mon, 13 Jul 2026 13:32:26 GMT

I do not have that lens, but I do use Extender EF 2X III. All of my older lenses work better on my EOS R5 than they did on older cameras. An old lens will not be worse and will likely be better than it was on the older camera, but still not as good as an expensive new lens. https://support.usa.canon.com/kb/s/article/ART173786 is a video about the manual focus assist guide in the EOS R.

With my EOS R5 and adapter, I have used EF100-400mm f/4.5-5.6L IS II USM +2x III and it seems to me to work very well, better than on my EOS 80D.
I have also used a Kenko TELEPLUS HD C-AF 2X DGX and Vivitar SERIES 1 1.4X AF with older lenses.
https://www.rsok.com/~jrm/2021Sep08_birds_and_cats/2021sep02_bumblebee_IMG_3837c.html
A Bumble Bee (Bombus pensylvanicus) was on a Zinnia in Norman, Oklahoma, United States on September 2, 2021. I made this photo with a very old lens as an experiment. I purchased the lens in 2011 and nearly wore it out. For this photo, it was attached to a newer camera body with an adapter with two telephoto extenders.

On a cloudy day,

https://www.rsok.com/~jrm/2026Jun21_birds_and_cats/IMG_6045c_2026jun18_egret.html

Snowy Egret (Egretta thula) at Lake Thunderbird in Norman, Oklahoma, United States on June 18, 2026

This might answer the specific question that you asked.

https://www.dpreview.com/forums/threads/100mm-300mm-5-6l-with-extender.220412/

Re: Extender EF 2x III compatible with EF 100-300 f/5.6L on EOS R?

deebatman316 — Mon, 13 Jul 2026 14:07:13 GMT

@rickmp001 I would take the advice from my colleagues @shadowsports & @Waddizzle that lens was Canon’s first EF L series lens. There are no lens correction profiles for it at all. Even from Canon’s own DPP photo editing software. The lens lacks IS and uses the original EOS AF Motor called AFD (Arc Form Drive) this AF motor was in production from 1987 to August 1991. It was phased out early on because it’s AF motor speed and the slow communication speed. These old lenses are classified under DPAF as F/8 lenses. Even though the lenses all have an aperture wider than F/8. These lenses also cannot AF & stop down at the same time. Since the communication speed is so slow. When the lenses are used on an EOS 35mm SLR, DSLR or adapted to the EOS R series. The FPS in Servo AF (formerly called AI Servo was locked down to 3-4 FPS). This is NOT a software limitation but a hardware one that CANNOT be changed. It may focus better on Mirrorless but the hardware is too limiting with that lens.

Re: Extender EF 2x III compatible with EF 100-300 f/5.6L on EOS R?

shadowsports — Mon, 13 Jul 2026 14:17:11 GMT

If I'm not mistaken, the EF100-300 (any) doesn't support a TC. If you try to use it with an extension tube, you're just "frankensteining" the entire configuration more and losing Infinity focus.

Re: Extender EF 2x III compatible with EF 100-300 f/5.6L on EOS R?

ebiggs1 — Mon, 13 Jul 2026 14:17:41 GMT

"This is not on the compatibility list, but I was wondering if this combination would work, ..."

I have no idea since I have never tried that combo. One main reason is, I would never try that combo if I had the opportunity. But that is my personal thoughts and the bottom line is what are you good with as a resulting image. If you like it and its seems OK then do it.

I went through the old lens to new and found out the images were not up to my standards. BTW, if you are adept at Photoshop or perhaps another editor you can make almost any image look pretty good I.E., the bumble bee.

I can say without hesitation the CA will be horrible. Again PS can help just too much effort needed to use that old stuff, IMHO.

Re: Extender EF 2x III compatible with EF 100-300 f/5.6L on EOS R?

deebatman316 — Mon, 13 Jul 2026 14:29:08 GMT

@shadowsports you are 100% right about that. The EF Teleconverter was originally designed for the original EF 300mm F/2.8L USM lens from 1987. Also the 1.4x Teleconverter was dropped alongside that lens. The 2x Teleconverter was released in 1988.

Re: Extender EF 2x III compatible with EF 100-300 f/5.6L on EOS R?

rickmp001 — Mon, 13 Jul 2026 19:46:17 GMT

Thank you for your reply. I am aware of the pitfalls of this lens, but for now it will suffice until I can justify funds for a more modern replacement.

Regards,

Rick

Re: Extender EF 2x III compatible with EF 100-300 f/5.6L on EOS R?

rickmp001 — Mon, 13 Jul 2026 19:49:03 GMT

Thank you for your reply. I realize the lens has many deficiencies, but until I can justify spending on a more suitable replacement, I'll live with it.

Re: Extender EF 2x III compatible with EF 100-300 f/5.6L on EOS R?

rickmp001 — Mon, 13 Jul 2026 19:52:00 GMT

Thank you for your reply. As this is the very first "L" lens in the EF lineup, I realize its limitations, but until a more suitable replacement is in the cards, I'll live with it and try to work on image correction.

Re: Extender EF 2x III compatible with EF 100-300 f/5.6L on EOS R?

March411 — Mon, 13 Jul 2026 20:12:10 GMT

@rickmp001 wrote:

Thank you for your reply. As this is the very first "L" lens in the EF lineup, I realize its limitations, but until a more suitable replacement is in the cards, I'll live with it and try to work on image correction.

It's not on the compatibility list and others have indicated it will most likely not work but it sounds like you've made up your mind to try it. Please post the results you get so that other members can use it for reference in the future.

Re: Extender EF 2x III compatible with EF 100-300 f/5.6L on EOS R?

johnrmoyer — Mon, 13 Jul 2026 20:52:29 GMT

@rickmp001 wrote:

Thank you for your reply. I realize the lens has many deficiencies, but until I can justify spending on a more suitable replacement, I'll live with it.

Not that much editing is needed to compensate for the extender. The extender will magnify any flaws in the lens. An old lens might work better for black and white instead of color. The editing will be unsharp mask and if the aperture is too small then Richardson/Lucy deconvolution. I usually do those for any photo.

I suggest using the lens in manual mode and take advantage of the manual focus assist in the EOS R. The Kenko TELEPLUS HD C-AF 2X DGX is much less expensive than the Canon extender and will work with many EF and EF-S lenses. If cropping, the image is reasonably good for the price. There will be some vignetting. Small aperture diffraction blur will be a problem unless one does Richardson/Lucy deconvolution. Rawtherapee free software does Richardson/Lucy deconvolution in "capture sharpening".

https://www.cambridgeincolour.com/tutorials/diffraction-photography.htm has a "diffraction limit calculator". When using it, use the F Number set manually on the lens multiplied by 2 for the extender. Select "Full Frame" for the EOS R. This will give "diameter of Airy disk". Check the spacing of photo sites on your sensor in the camera specifications at Canon. Divide the number for Airy disk by 3 and divide by photosite spacing. This is the "radius" to use in Rawtherapee capture sharpening. The number of iterations can be larger if low ISO and little noise, but must be smaller if there is noise or non-Gaussian blur and distortion present. The Richarson/Lucy deconvolution does very well at removing small aperture diffraction blur if the distortion in the image does not deviate too far from the Gaussian approximation or if one does not do too many iterations. A wrong radius will result in ringing at edges (halo) when one does many iterations.

https://en.wikipedia.org/wiki/Airy_disk#Approximation_using_a_Gaussian_profile has the calculation. A wavelength for green light is a good approximation because there are twice as many green pixels than either red or blue.

I made this table for myself for my EOS R5. I used 0.52 as the wavelength, but 0.55 is more common as an approximation for green light. Instead of the 0.84 and 1.22 mentioned in the Wikipedia article, I used 2/3 of 2.44.

In rawtherapee, capture sharpening, contrast 1, 20 to 60 iterations, Richarson/Lucy deconvolution

For EOS R5, 4.39µm pixel spacing on sensor, wavelength 0.52

Airy disk size is 2.44 * 520 *F Number

F Airy pixel radius for rawtherapee

F/8 10.1 / 4.39 / 2 / 3 == 0.38

F/9 11.4 / 4.39 / 2 / 3 == 0.43

F/10 12.7 /4.39 / 2 / 3 == 0.48

F/11 14.0 / 4.39 / 2 / 3 == 0.53

F/13 16.5 /4.39 / 2 /3 == 0.63

F/16 20.3 / 4.39 / 2 /3 == 0.77

F/22 27.9 / 4.39 / 2 / 3 == 1.01

https://en.wikipedia.org/wiki/Unsharp_masking

Re: Extender EF 2x III compatible with EF 100-300 f/5.6L on EOS R?

rickmp001 — Tue, 14 Jul 2026 02:02:34 GMT

Using a crop sensor only has effect on angle of view, not magnification/image size.

Re: Extender EF 2x III compatible with EF 100-300 f/5.6L on EOS R?

rickmp001 — Tue, 14 Jul 2026 02:07:14 GMT

Thanks for your reply. I was merely looking to see if it was even mountable without sustaining damage, not as a serious solution to imaging. I’m fully aware of this lens being the most ancient L lens in existence; it was merely a budget solution for a long focal length zoom lens. Ideally, if the resources were in the budget, I’d look for a more modern lens.

Re: Extender EF 2x III compatible with EF 100-300 f/5.6L on EOS R?

rickmp001 — Tue, 14 Jul 2026 02:10:04 GMT

Thanks for posting the images. I realize that lens is the most ancient L lens in existence. Looking to replace it eventually with a more modern lens.

Re: Extender EF 2x III compatible with EF 100-300 f/5.6L on EOS R?

ebiggs1 — Tue, 14 Jul 2026 14:19:29 GMT

"...until I can justify spending on a more suitable replacement, I'll live with it."

The lens you have is always better than the lens you wish you had.

Re: Extender EF 2x III compatible with EF 100-300 f/5.6L on EOS R?

rickmp001 — Tue, 14 Jul 2026 22:42:39 GMT

Hehehe, no truer words were ever spoken. With this particular L zoom, as it’s the oldest existing L lens, any more modern lens would have its advantages. While the EF 100-300 f/5.6L has its warts and is rather long in the tooth, it can be used to produce some good images, but its ease of use and image quality is not up to the standard of the also not-so-new EF 70-300 f/4-5.6 L IS USM or the EF 100-400 f/4.5-5.6 L IS USM. I used to be a pro shooter many years ago, but didn’t shoot 35mm (mostly 4x5 in a studio) but now, I’m more of a weekend warrior and need to save my pennies (remember them?) to raise funds to replace the oldest L lens in the world. I was hoping to get feedback whether even mounting the Extender 2X III would be risking damage and the general response seems to say not to bother trying it. Thank you for your response.

Regards, Rick

Re: Extender EF 2x III compatible with EF 100-300 f/5.6L on EOS R?

rickmp001 — Tue, 14 Jul 2026 22:52:32 GMT

Excellent response, definitely something I can sink my teeth into. Rawtherapee is a cool piece of software. DPP is good, I never kept up to date with Adobe’s offerings, but even the basic iPad version of LR has its points. Thanks for posting, good info to look into. Regards, Rick

Re: Extender EF 2x III compatible with EF 100-300 f/5.6L on EOS R?

rickmp001 — Tue, 14 Jul 2026 22:57:46 GMT

It’s definitely not on the compatibility list, have not determined I was going to give it a try necessarily. Was hoping maybe someone had tried using that combo and could report that it didn’t result in any damage and if that was the case, I might make an attempt. Even the oldest L lens in the known universe that is working is better than one damaged by attempting to mount it to an incompatible extender. Thank you…