One of the best combinations was aways with the Canon EF100-400 and a 1.4x converter, or with a Sigma / Tamron 150-600mm lens.
I think your comment about the R5/R6 being superior performers is very valid, if the cameras are setup correctly. Out of the box, they are dauntingly complex and many are returned to dealers because they are too intimidating.
A bit like the 7D2 "
was" is the key word when it comes to the 100-400 Mk 2 plus 1.4TC. A superb lens that with the TC gives you 140-560mm of reach. Better still with the R5 and presumably the R6 and R3 which I have no experience of, allows full AF point coverage across the sensor. Better still, with a 2.0x you have 200-800 with full AF point availability too despite it being f11. (You can't do that even on a 1DX2 unless you shoot live view which is far from satisfactory)You are of course compromised in speed and light but it's doable.
The "
new" kid on the block is the RF100-500 which gives almost as much reach at 500mm but without a TC. Again, light and AF speed is compromised with a 1.4TC ( I haven't tried using a 2.0x and probably won't ) but you have 700mm at f10. A better option than the Sigma too in my opinion. It's biggest design fault for me is that if you stick a TC on the lens it becomes a 420-700 zoom. Fine for most birding situations but not necessarily for bigger creatures or sports.
The cheapest...let's re-phrase that....the least expensive option for long reach is undoubtedly the 800mm f11 but that isn't without it's drawbacks too.
The field of view is narrow, it's a prime so obviously not as flexible as a zoom and the construction, although clever, is not as robust as "L" class lenses.The minimum focus distance is quite long and it comes without a lens hood or a tripod foot. The latter I find disappointing not because I want to mount it on a tripod but it lacks an obvious way to carry it around. F11 is light challenging although it copes better than I thought because the IS and higher ISO abilities of modern cameras allow you to use lower shutter speeds. That said wildlife is prone to move so that negates the latter advantages. No, the 800mm is an excellent buy for the budget, it is capable of very sharp images but buyer beware. Attached to an R body you only get to use a centre block of about 25 % of the sensor coverage. I'm sure talented photographers can cope but I feel challenged that I can't use the AF points and eye detect to compose some shots as I would like.
Out of interest, and for me as I get older the weight is critical, the R5 plus single battery and the 800mm weighs in at 2.055kg. Light as a feather! With a 1.4 TC it weighs 2.285kg but at f16 most definitely light challenged.
The 100-500 with and added QR plate for a carry handle with R5 etc weighs 2.5kg, add the 1.4 and it weighs 2.73kg. In my opinion by far the better buy but it comes at a cost but there again most things do and it's a case of compromise or budget.
The R5 etc with an EF-R adaptor plus 500mmf4 Mk2, replacement lightweight QR tripod foot and 1.4TC comes in at a fairly hefty 4.8kg. Fine for younger bodies but mine struggles to cope carrying it too far nowadays but what a magnificent combination it makes!
I bought the R5 over the R6, after much consideration I might add, and I have no regrets. It's expensive, and yes I had to buy extras to go with it. CFexpress cards, a bigger SD card, extra batteries and a new desktop which I was due anyway and probably in due course some more external hard drives. Of all the camera bodies I have owned including 2 Nikon and 7 Canon DSLR's right up to the 1DX2, this is the biggest leap in technology by far.
This body brings to the table what the 7D2 probably did on it's launch. I can't see needing an upgrade for years to come by which time I'm sure those who are new to the game will be wondering what an earth is the old codger doing with that bit of ancient rubbish. He needs to upgrade to the R23 not the R12 at a fraction of the price. I will hopefully still be taking pictures purely for the pleasure of it though and for me the thrill is, and always will be , being there to take the shot and hopefully get a reasonable image in to the bargain. I could still possibly get that on a 7D2 and saved a fortune on all those 1 & 5D's I have owned and sold but for the moment 45mp and eye detect rules the roost!!
Shrouds don't have pockets as they say!