800mm advice

I'd like to purchase 800mm lens this year. I'm not willing to spend $16k+ on Nikon lens
and I figure it'll take 2-3 years until price will come down to $10k on second-hand market.

Looking at some ebay sales I see that I could get Canon 800mm plus 1dx mark II for roughly $10k.

I'm not familiar with Canon system at all and my biggest lenses are Nikon 600mm G VR and Sigma 300-800mm.
Both are fine but NIkon with 1.4 converter even with f8 is kind of soft and Sigma at 800 is soft too.

I'm planning on using 800mm alone and with 1.4 EF III extender.
Anything I should be aware of with Canon 800mm and 1dx mk2 ( I'm aware of small number of af points)?
Any tips or comments appreciated.

The Nikon 800mm with its dedicated 1.25x converter is in a league of its own - the older Canon 800mm is no comparison.
For a review refer to here.
If you're not happy with the Nikon 600mm plus TC14 - my suspicion is you will not be happy with the Canon 800mm plus converter either.
I would rent both before buying - but - you may just have to save up for the Nikkor lens.

There's no inexpensive way to get to get a quality 800mm right now. The Canon 800L isn't a bad lens by any means, but it's not up to current standards either. IMO you'd be better off working with your 600G+TC14EIII+D850 and cropping than you will be with the 800L+ 1DXII.

There is simply no comparison with the now very old Canon lens, which is still based on their first generation AF lens design principles, and the Nikon lens.  I have shot with and tested both, they are not in the same league.  Similarly if you are using a current Nikon body like a D500, D850, or D5, nothing Canon currently makes is in the same league for autofocus.  Now all that may change as we approach the Tokyo Olympics in 2020, but for now that is the state of play.

I sold my canon 800 for the canon 600II. The 600ii is more flexible and if memory serves me right is generally considered sharper with the 1.4x at 840mm than the straight 800. As to longer configurations 1200 vs 1120, the digital picture has some test target comparisons for you to review. most of what I read indicates no real sharpness difference between the version II and III. The improvements seem mainly weight, IS and MFD.

In all honesty the Canon 800mm is well overdue for an update and by no means a good reason to change camera systems. The better option would be the 600mm (either mk2 or 3) with a 1.4 TC. I chose the 400 2.8 as I need 800mm less than 400 and 560 but when I do use a 2x TC I am constantly amazed.

Andrew_5488 wrote:I'd like to purchase 800mm lens this year. I'm not willing to spend $16k+ on Nikon lens
and I figure it'll take 2-3 years until price will come down to $10k on second-hand market.

Looking at some ebay sales I see that I could get Canon 800mm plus 1dx mark II for roughly $10k.

I'm not familiar with Canon system at all and my biggest lenses are Nikon 600mm G VR and Sigma 300-800mm.
Both are fine but NIkon with 1.4 converter even with f8 is kind of soft and Sigma at 800 is soft too.

I'm planning on using 800mm alone and with 1.4 EF III extender.
Anything I should be aware of with Canon 800mm and 1dx mk2 ( I'm aware of small number of af points)?
Any tips or comments appreciated.

  Not sure why you think a used 800mm lens will come down to $10K in a couple of years, it has been out for quite a while and I have not seen one that low (maybe if Nikon comes out with a new lighter 800mm lens?).  I share the view expressed that you should rent one, I did a couple of times and was super impressed.  However don't think you want to use it with a 1.4TC (I actually tried it) but it does come with a dedicated 1.25TC that it works well with.  I too can't justify the purchase price so have stuck with the 500mm f/4 plus a 1.7TC when I have to.  Not as good as the 800mm for sure but one has to ask the question of what is a picture worth to you?  If it is worth the dollars, there is no equivalent of a Nikon 800mm (at this time).