alberio Loc: Casa Grande AZ

My EF400L lens is a older non IS f5.6, but pretty good in my opinion. I'm wondering if I got the 2xIII extender, how would the performance compare with the Canon RF 800 f11?

CHG_CANON Loc: the Windy City

The native prime should deliver better overall results, simply for the fact of less glass in the resulting configuration. Being a newer native mirrorless design also should exhibit better results. The IS of the RF lens is additive to the IBIS of those enabled bodies. The 2xIII is a $200 to $300 experiment, where if you're dissatisfied, the resale market is low, especially for the less-compatible 2x. You might find these differences are immaterial, where the old is still just as good, but I'd only engage in that experiment if I already owned all the equipment involved.

alberio Loc: Casa Grande AZ

Thanks Paul. I do have the 400 and also a pretty good 70-200mm f4L and thought it might be an additional option. I've now seen some photos with the RF800 and in good light, it does pretty well, but also more money.

JFCoupe Loc: Kent, Washington

Go to the following web link, a Canon specific site and ask your question there. There maybe someone with experience with both lenses.

https://photography-on-the.net/forum/

LEWHITE7747 Loc: 33773

I use the old 400 prime with my new R7 with incredible results. The IBIS makes a difference with the lens have no IS.

PHRubin Loc: Nashville TN USA

I would expect that an f/5.6 lens and 2X extender would result in an apertue (~f/11) that would be beyond the camera's ability to use for focusing.

CHG_CANON Loc: the Windy City

Not on today's EOS mirrorless options. That's why there are now RF f/11 lenses that can also be extended.

Orphoto Loc: Oregon

The alternative OP is considering is also f11.

alberio Loc: Casa Grande AZ

I would mostly be using manual focus dedicated tripod use, not sure if auto focus would be used much. More interested in optical performance between the two.

imagemeister Loc: mid east Florida

I have the 400L prime and have used it extensively with the 1.4X II. I also have the Canon "Lens Work" books than show the MTF charts of the 400 with the 1.4 and 2X II's. I have also seen the Imatest numbers for the 800 f11. If I were willing to work with a f11 aperture, I would opt for the RF 800. The EF 400 is a very GOOD lens but the 2X cuts it down heavily ! - but still may be "satisfactory" for most people. And, I have seen no convincing evidence that the version III TC is any advantage over the version II for the older EF400 ......
.

alberio Loc: Casa Grande AZ

Thanks for that info. I'll look into it as well.

amfoto1 Loc: San Jose, Calif. USA

BUT...

There is nothing to indicate the original poster is using an EOS mirrorless camera.

In fact, the most recent image they posted was shot with a six or seven year old 5DS DSLR. That certainly won't be able to focus an EF 400mm f/5.6 with a 2X added.

OTOH...

The original poster is aware of and commented about the RF 800mm f/11, so maybe they do have one of the R-series cameras.

Regardless what camera it will be used upon, my main concern would be image quality. Any time a teleconverter is added to a lens there is some loss of IQ. The most premium lenses like the EF 300mm f/2.8, 400mm f/2.8, 500mm f/4 and 600mm f/4 are so exceptionally sharp to begin with, they can "tolerate" some loss with teleconverters. While the EF 400mm f/5.6 is a fine lens in it's own right, it's not on the same level as those super teles, so is likely to show more loss of IQ. Further, there is considerably more loss with a 2X than with a weaker 1.4X teleconverter. Maybe give the 2X a try, but be prepared to exchange for the 1.4X.

I don't have that particular lens, but do have the earlier II version 1.4X and 2X (which are very similar to the III version extenders). I use the 1.4X on quite a few lenses: 135mm f/2, 100-400mm II, 300mm f/4, as well as 300mm f/2.8 and 500mm f/4. I only use the 2X on the 300mm f/2.8 and 500mm f/4. For my purposes, there is just too much loss of IQ with any other lenses in my kit. But, again, I haven't got the EF 400mm f/5.6, so can't say how they work on that specific lens.

rbtree Loc: Shoreline, WA, United States

Get a 2x III. They can be found used for well under $300. Or a II as I don't recall whether the III performance is better on older glass than the II.

That venerable 400 is a Canon "Big White" that I've never owned. But it's a great lens, as you know.

I often use my two Canon Extender III's on any of my tele lenses. The IQ and sharpness never disappoints.

OP, you didn't mention if you have a mirrorless body as well?

Architect1776 Loc: In my mind

Not the mirrorless.
The 800mm f11 the OP mentioned AFs just fine.
My EF 100-400mm L MII and 2X III has no hunting or AF problems adapted to the R7.
That combo is f11.

alberio Loc: Casa Grande AZ

I have an R6, and yes a 5Ds and a 7DmkII.