R5 lunar photography with stacked EF extenders

Bigger wrote:

There was some interest in Stacking RF extenders in an older thread that focused a lot on maximum focus distance. So, I thought I would start a new thread on stacking EF extenders on RF bodies for lunar (and planetary) photography. Here's a sample image taken this evening:

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Based on my testing, the EF600/4 II has resolution to spare with the 2x converter as evidenced by aliasing a Siemens star beyond the Nyquist limit, so it's a good candidate for stacked extenders. Not that this sample is demonstrating any kind of technical excellence, but I think it shows the lens holds up well with stacked extenders. It also shows how the moon fills an APSC frame at 1680mm.

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Lunar Photography with Extenders...
I wouldn't mind an EF 600 f/4L II lens although I wouldn't use it enough to warrant the cost... but it's considered to be a heavy lens and one with respected optics. I've considered the EF 800mmL as well, although I might wait to see what Canon unfurl with newer lenses as they are released in the near future.  That said, I'm considering the RF 600mm simply because it's ready to mount on the RF-system.
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I bought the EF 100-400mmL II lens as a replacement for a telescope and I've been shooting with the EF 100-400mm f/4.5-5.6L IS II USM lens coupled with the EF 1.4x III and EF 2x III Extenders for quite a few years now. When it comes to lunar photography, I've generally been happier with an APS-C mirrorless body because of the 1.61x Crop aspect which tends to magnify the size of the moon dramatically. But with fuller moon-phases, the more light you have to play with. A problem with shooting near-full moons is that you don't get as much crater shadow detail and this can have the effect of producing a flat-looking lunar disk. You can still get beautiful shots of a full moon with any camera and framing it with cloud or a landscape can also have an impact with the image.  The EF 100-400mmL II lens is known for its sharpness and that's one of the reasons it's filling in for me as a telescope at the moment.   Every time I put the money together to buy a dedicated OTA, the prices rise dramatically and something more important pops up.
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EF12 Tele-tubes...
Not the Extenders, but the Tele-tubes... I first tried the EF12 clone from a brand called Pro-Master. It was all I could find locally at the time. I believe that these are simply rebranded Kenko tubes and, as a result, the units are VERY lightweight compared to the Canon EF12. Every time I went to buy a Canon EF12 they were out of stock and only very recently did I get my hands on the genuine tele-tube. The difference in the fit is much better with the Canon. The Pro-Master was looser with considerably more play. To my horror, picking my lens up with camera and extenders and Pro-Master tele-tube resulted in the tube uncoupling from the camera end, resulting in a very near accident. The cause was that my little finger touched the Lens-decouple Lever which is incredibly loose on the non-Canon branded tubes.
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The EF Mk II Extenders were made for 35mm film cameras and Canon don't recommend them for Digital cameras due to inconsistencies with exposure and AF.  When Canon made the older Mk II Extenders you could mount them back-to-back and double-stack them together.  Canon felt that people were constantly doing this and then complaining about image quality ...so when they released the EF MK III Extenders, they made some physical changes that prevented the mounting of two extenders together.  This is why the EF12 Tele-tube is handy and why it's required to double-stack the Extender lenses.  You can also use it to convert other lenses into Closeup lenses.
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Some image samples below from my own cameras/lenses/extenders.  The images were all shot in JPEG and now that I've replaced my computer I'll consider working with RAW images in future.  I might even get around to stacking something some day soon.

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EOS R6 + EF 100-400mmL II + EF 1.4x III + EF12 + EF 2x III Extenders.

Lining up the moon with the EF 2x III Extender attached to the EOS R6

One of the Extenders is always stripped from the EXIF data, which is one of the reasons why Canon don't recommend using two Extenders back-to-back.

R6 + EF 100-400mmL II + EF 1.4x + EF12 + EF 2x III Extenders during a lunar shoot.

R6 + EF 100-400mmL II + EF 2x III Extender (cropped for detail).

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Impact of Extenders on Image Quality...
Using non-Canon brand Extenders is simply a waste of money.  And the older Mk II Extenders are pre-digital and will soften the images.  The more recent EF Mk III Extenders were redesigned "from the ground up".  They are optically excellent and are more robustly designed with a faster microprocessor.  EF Mk III and RF Extenders offer the same image quality and this has been confirmed by numerous photographers with access to both.  But since magnifying an image will magnify any shortcomings of a lens as well as the atmospheric influences, the EF 1.4x III Extender (and RF 1.4x Extender) will produce very little impact on image quality whilst offering the lest amount of light reduction.  A 1.4x Extender is said to reduce the aperture by 1.4x by default.  The same applies to a 2x Extender which cuts light transmission by around 2x.  So you'll likely be shooting with either f/11 or f/13 with the lens extended at 400mm.  They say you should consider reducing the aperture by a small increment when using Extenders but that's going to cost you even more light.
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I enjoy using extenders although with this particular lens I prefer to either shoot with Just one Extender where possible.  The detail from the EF 1.4x III alone on the EOS R6 is great.   It offers plenty of light transmission and sharp images.  I also like the EF 2x III for lunar shots with a Full Frame camera where I want a larger moon, or the EF 1.4x III if scale is less important.  With an APS-C camera I can make out the rings of Saturn in daylight with just the 2x III Extender on the lens at 400mm.  The EF 2x III produces surprisingly decent results by itself.  Adding both Extenders usually prevents the lens from focusing at "100mm" (the moon simply becomes a blurry blob) but you can do it if you remove one.  yet there's sometimes a need to want to push further by stacking both Extenders together.  On a Canon APS-C camera with a 400mm lens, that results in 1803.2mm (*equiv) and on a full frame camera like the R5 or R6, you're getting 1120mm of effective focal length. You certainly fill the frame with the moon when shooting with APS-C at 1803.2mm, and any shots with Full Frame result in an occasional need to crop a little.  But not always.
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R6 + EF 100-400mmL II + EF 1.4x III + EF12 + EF 2x III Extenders. Cropped for detail.

R6 + EF 100-400mmL II + EF 2x III Extender - set up for the shot below

EOS R6 + EF 100-400mmL II + EF 2x III Extender.
Super Moon rising over Sydney's bushfire smoke - Cropped to remove a tree.

The difference between APS-C and Full Frame camera sensors at 800mm (uncropped)

EOS M6 + EF 100-400mmL II + EF 1.4x III + EF12 + EF 2x III Extenders.
This was shot handheld. Yes, the OIS in the lens is that good.
*Cropped & rearranged for detail.
** A second "overexposure" was required to see Jupiter's moons.