R6 with EF 100-400mm II and TC EF 1.4x III for wildlife?

XN_Stein wrote:

Hi,

Would like to get a R5, but don't really have the budget for it. 🤣I would like to ask if anyone had used the EOS R6 with the EF 100-400mm II and TC EF 1.4x III for wildlife before. May I know how is the result? Will this combination be long enough for most shot?

I do, and it's a VERY good combination. In fact you get such a great response from this very combination that you may do what many others do and just leave the Extender on the lens most of the time. You'll basically get a 140-560mm with excellent image quality throughout the zoom range with the EF 1.4x III Extender attached. This particular Extender is also portable enough to fit into many pockets so it can be carried as an optional accessory if you like. AF speed it great.  As for if 560mm is long enough for your preferred shooting, only your subjects and shooting preferences are going to answer that.  Looking at examples from 500mm lenses this is more than enough focal length before other factors get in the way.  The images without the Extender at 100-400mm matched those from the RF 100-500mmL and side by side we found them indistinguishable from one another.  Whilst the RF 100-500mmL lens has an extra 100mm to play with, it's on the far end and the aperture is reduce much sooner in the zoom range than the EF lens permits.  Adding an EF 1.4x III Extender gives you an impressive 560mm to work with. I found this distance to be ideal when photographing animals in the wild and also in zoos during visits last year.  There's an image of a wild rabbit in a park below that I've posted and whilst I was able to creep up close to it, there's a limit as to how close you can get to these critters.  The Auto Focus is fantastic with this lens although I usually prefer to mount a collapsible monopod to the lens ring foot and that allows me to shoot for long hours as needed without fatigue in my arms.
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BG-R10 Battery Grip (optional and unnecessary).
You DON'T need to buy the BG-R10 battery grip for use with this combination, but the shifting of weight by adding the BG-R10 can offer you more grip surface area for your hands and it will allow you a surprising amount of shots with two batteries. I got close to 7,000 shots on a beach in a single day using the grip.  But the R6 does NOT need the grip and it will perform very well without one.  I spent months waiting for my own BG-R10 to arrive and the difference in performance and handling was minor.  In fact I liked using it without a grip when using this lens combo.
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Eye Defect AF performance on the R6 + 100-400mmL + EF 1.4x III
With the Animal Eye Detect-AF selected, you can select a few different options for AF methods... and you can force the Eye-detection to become active whenever you press down one of the back-buttons on the R6 camera ...and this makes is a pleasure to use when photographing just about any type of animal, bird, lizard etc. The weight isn't too bad although holding this combo for over 10 minutes in front of you with your arms extended would be problematic if it wasn't for the shoulder strap or the ability to affix a Monopod to the lens foot. When photographing at the zoo or shooting wildlife in the open, you can easily lift the lens for brief periods but a Monopod makes this so much easier to nail the subjects and to hold the camera in front of you for long periods of time. 
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EOS R6 + EF 100-400mmL II + EF 1.4x III Extender

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Even without the Extender, this lens is a real gem all by itself. It's sharp, reliable and robust. If offers great performance with sharp images and extremely flattering bokeh.  I would advise using a UV Filter (77mm) on the front to seal the unit in dusty or moisture laden environments. A chap I know on Facebook scratched off some of the optical coatings by repeatedly scraping his plastic lens cap on the front element over months or even years.  Circular Polarizers don't seem to vignette at all on this lens and banding isn't an issue, although using really cheap (eg $5) UV filters is not recommended. The Image Stabilizer on this lens has quite a reputation for being extremely effective.
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The EF 1.4x III has no negative impact on your image quality and the drop in AF speed (which is determined by a microprocessor inside the Extender) is not enough to be noticeable. It's still so fast that most other photographers don't notice it.  I was able to track the eyes of a young baboon after its older sibling grabbed it and ran off with it... which surprised me.  The only subject I had to manually focus on was the eye of a Green snake which you can see in the pictures I posted below.  I simply placed an AF reticule over the snake's eye and used the Auto Focus without the Eye-Detect active.
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Microcontrast is excellent. Color rendering is excellent. Image quality was the same as the RF 100-500mmL lens when I used the two side by side shooting in the mountains when the new lens was released. I ended up cancelling my RF lens order although I'd buy one to replace my EF lens if it was damaged in future. I stuck with the EF 100-400mmL II lens because it offers me more light (via a wider aperture) when zooming out throughout the zoom range and this is sometimes useful for me when shooting in low light or with astrophotography. 
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I use this combination of R6 + EF 100-400mmL II lens + EF 1.4x III Extender for surfing photography and especially wildlife. You also get a great closeup capability with this lens. The EOS R6 has fantastic low-light capability and uses the same AF system as the R5. I find that the noise from the R6 is usually only noticeable when shooting in low light with 100 ISO. In bright daylight 100 ISO is not an issue but it can produce grain in lower light environments. ISO 400 and ISO 800 are great for low noise. Even ISO 6400 can be very low noise on the R6.
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Compared to the EOS R7 as an alternative?
I'd normally suggest an APS-C sensor for additional "zoom" with any long lens although shooting wildlife requires reliable critical focus.  But I haven't been overly impressed with the new EOS R7 and for tracking birds in flight there's been some issues raised recently.  I've used this very lens and Extender on the EOS M6 (APS-C) and was extremely pleased with the results, though I wasn't demanding Eye-Detect AF on that camera.  But the R7, despite having a much better AF system than my EOS M6, is not in the same ball park as the EOS R6 when it comes to reliable AF performance.  There's a few videos on YouTube that explain what others are seeing.  Hence, I'm not going to recommend it with this combo.  The EOS R7 has performance issues with both lowlight performance (probably due to the smaller APS-C sensor), noise (high ISO is often required at longer focal lengths and the resulting grain is inevitable) and it also has an issue with AF accuracy when tracking moving subjects whilst panning (causing the focus to jump back and forth between the background scenery and the subject). This is not an issue with the EOS R6. I'm only mentioning it because the performance is notably better coming from the large Full Frame sensor of the R6 which also makes good use of the sensor's real-estate with larger pixels.  The higher Dynamic Range and -6.5EV on the EOS R6 is also preferable and it gives you better performance on the long end of the zoom with an extender and even a filter. The larger, 'brighter' sensor on the EOS R6 will allow you to use higher shutter speeds with lower ISO settings with less noise and better overall performance.
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A lot of people like to photograph the moon when they get a long lens and you can certainly see the craters on the moon with this lens + extender combo.  If Astro and Lunar photography are something that interest you, Full Frame isn't the ideal sensor size for this since the moon is still fairly small... and APS-C will give you more reach and will add another 1.6x "magnification" (via the sensor crop).  Using the 2x III Extender with this lens and an APS-C sensor will give you a lot more detail on the lunar surface.
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EF 1.4x III compared to EF 2x III on the EOS R6
(Just in case you were wondering).  I use both but rarely use the EF 2x III Extender. The 1.4x III Extender is more flexible, easier to use, performs faster mechanically and offers better image quality due to the lower magnification compared to the EF 2x III Extender.  The 2x III also produces less contrast which, when added with a 75% speed reduction, will hamper use unless you are in a fairly bright location (beach, park or zoo on a sunny day).  If the subject is fairly close to you, the 2x III Extender will produce clean images with plenty of detail, even near macro closeups are possible.  But if the subject is farther away, especially quite a distance away, the EF 2x III produces "smoother" looking images in low light due to contrast (or lack thereof) and subjects a LONG way away may produce softening due to thermal fluctuations in the air.  This is not an issue with the EF 1.4x III Extender.
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The samples below are from my own camera and were all shot in JPEG with additional tweaks as needed after downloading.  If this is a setup that meets your budget, it's a very good combination for wildlife, landscapes, action and sporting. 
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EOS R6 + EF 100-400mmL II + EF 1.4x III Extender + UV + CPL filters

EOS R6 + EF 100-400mmL II + EF 1.4x III Extender (partially cropped for detail)

EOS R6 + EF 100-400mmL II + EF 1.4x III Extender + UV + CPL filters (wild rabbit)

EOS R6 + EF 100-400mmL II + EF 1.4x III Extender + UV + CPL filters

EOS R6 + EF 100-400mmL II + EF 1.4x III Extender + UV + CPL filters

EOS R6 + EF 100-400mmL II + EF 1.4x III Extender + UV + CPL filters

EOS R6 + EF 100-400mmL II + EF 1.4x III Extender + UV filter

EOS R6 + EF 100-400mmL II + EF 1.4x III Extender + UV + CPL filters

EOS R6 + EF 100-400mmL II + EF 1.4x III Extender + UV + CPL filters