[Klaus]

https://www.opticallimits.com/canon_eos_ff/1170-canonrf800f11

Decent ... but ... slow.

[Rover]

OR should we say dim.
But yeah, I expected the resolution to be a lot lower, not the least given the aperture value.

[stoppingdown]

« at a distance of 25mm »?

[davidmanze]

Canon's taken the cheap route (F11) ...... Nikon the moderate price route F6.3 .....
  
  ... Goldilocks is looking for an F8 version .......

[toni-a]

On paper its performance looks very decent, however I am almost sure it's a hassle to use and the quality we have is only on test charts where a tripod is used.
Used handheld either you will have to live with a lot of noise or suffer dramatic losses due to noise reduction

[Klaus]

You can check the sample images - all taken hand-held.

[toni-a]

You can check the sample images - all taken hand-held.

photos taken on a bright sunny day yet ISO 3200 was often needed....

[Klaus]

It doesn't really matter if the noise can be handled ...

But yes, in all but good light, it'll be difficult.

[davidmanze]

It's a lens which has attracted many utube videos since it's release, nearly all of which are in the field often on the R5 or an R7 (crop body which gives a FL of 12,800mm)

  noise "is a bone of contention" however in the reviews but everyone is using Topaz denoise these days for birding and the resulting images the lens produces are frankly very good indeed, often of small birds appropriately distanced to produce soft melted backgrounds that belie the images are taken at F11, using ISOs up to 12,800 or so in some cases ....
 So from my initial condemnation of this lens has somewhat changed, for a grand you get a light retractable 800mm lens with OS with a MFD that can give you "head and shoulder portraits" of small birds that even big boy glass can't achieve ........ and the thing weighs no more than your average pair of light trainers .......
 ........ and most comment how it's light and simple to use and carry ..

btw.  ....... I used to shoot at 600mm @ F11 using the Tamron 150-600mm G1 before getting the G2 version ....

[Klaus]

I think the only aspect to keep in mind here is the simple fact that you can at least get decently fast 600mm zoom lenses for reasonable money.
If I had to choose between the Canon RF 800mm f/11 and the Sony 200-600mm f/5.6-6.3, I, for one, would have a clear preference.
Of course, that's irrelevant within the Canon scope, but then Canon is a choice as well.