My point in comparison is that the Sigma 150-600 does an amazingly good job when compared to a lens that is so expensive that just the replacement carbon fiber lens hood for the Canon EF-800 is almost as expensive as the entire Sigma 150-600 contemporary lens. And it weighs over 5 pounds less than the EF 800 so I am definitely not criticizing the 150-600 in any way. Sigma is producing some incredibly good products these days. And I agree that you can never have too much FL with wildlife.
A couple of days ago, I shot a crop dusting helicopter for a friend. I showed up with three camera bodies along with Canon EF 70-200 f2.8, EF 800 f5.6, and the Sigma 150-600 glass (the Sigma was on a 1DX II body). Most of the photos were taken with the Sigma, it is incredibly versatile and focuses rapidly. The Canon primes are certainly somewhat better but trying to juggle camera bodies and lenses to switch focal lengths with a rapidly approaching or leaving subject will cause you to miss a lot of opportunities and this is where the Sigma zoom earns its keep.
The Sigma works well across its FL range, examples: the first photo was at 226mm, the second at 374mm, the third at 531mm and the fourth at the minimum 150mm focal length. The lenses and Canon bodies worked fabulously, my body would have worked better if there hadn't been a last second time change so I showed up without my normal morning caffeine content 🙂
Now if Canon actually made a 150-600, I would likely have been drawn to it primarily for the Canon brand name 🙂 But I have been using the Sigma far more than I expected and I am glad that I gave it a chance.
Given that you have not actually defined even a rough budget: "I can not afford high priced lens" covers a lot of territory. However, the following options are available to you:
Get a good, used Sigma 150-600c lens and use it with a Canon EF-RF adapter.
Pro's: It is a great lens, as many posts have indicated: relatively sharp, versatile, light and great zoom range
Con's: Being an EF lens it requires an adapter, and Sigma have issued a statement the lens is not designed to work with Canon's R7's bird eye tracking and servo autofocus. However, if you want a practical experiment with the Sigma 150-600c on the R7, then watch this video: HERE . Duade Paton is respected Australia bird photography specialist and has a very down-to-earth and no-nonsense approach to his videos - If you are keen on bird photography, I recommend you follow his posts.
If you watch the video you will see, and I confirmed that with him, that the autofocus pulsing really only happened as the lens approached its minimum focus distance.
If you decided to stick to an native RF mount zoom lens, then the most economical, with any decent reach, is the Canon RF100-400. It means you don't need an adapter, which saves a bit. Duade also produced a comprehensive review of the RF100-400 lens HERE - it's very in-depth. You may be able to pick one up from Canon's Refurbished Lens site HERE - keep an eye on the site as the offerings change.
Pro's: Native RF lens, makes use of R-series stabilization, focus tracking, combining lens stabilization with body stabilization features
Con's: not the same reach as the Sigma
Much depends on the conditions under which you shoot and, very significantly, what you intend to produce. The demands for producing images for the web or digital displays are much less than producing large, detailed, fine Art prints.
I can only report on how well they work on a DSLR as I do not and don't intend on buying a mirrorless rig. There is some hope for the reported eye detect feature failure as both the Sigma C and Tamron G2 are user FW upgradable, Apparently they both work on mirrorless but eye detect is only hit and miss at best. If you don't use the eye detect technology they seem to do quite well.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.