Is there a maximum practical reach?

As I ponder reach at 800MM and up, a 600 or 800 plus the TC(s), I wonder whether or not it is worth it when shooting wildlife and, if so, under what conditions.

What I'm curious about is the experience others have had with atmospheric conditions making a length 'not worth it' -- e.g. in Yellowstone, I have shot at 800, and the rising heat and the dust between me and the subject make the results mostly bad........

Thoughts?

BTW, my max is 800 configured as 400 f/4 IS DO + 2XTC; my choices will be 600 f/4 + 2X, or 800 f/5.6, so it boils down to this -- If generally shooting >1200 is going to be unsuccessful simply because conditions must be perfect to obtain a crisp result, then I'd go with the 600 at 1/2 the cost....

A long range lens beyond a doubled 400 has never graced my bag, but I do know of what I speak at long range with optics.

Shooting the big rifles with the big scopes at the long ranges has shown me what the prevailing conditions will do to light and how any vibration or unsteadiness will drive you to crankiness.

I am not sure what 1200mm relates to in terms of optic "powers" in scope terms, but I have seen similar effects in my viewfinders at 800mm and assume it only gets worse from there. Of course, we are trying to steady a shot with crosshairs directly on target, photography doesn't need to be that accurate, just quick enough to freeze action when the button is pushed.

yes 800 mm may be the limit for conventional dslr w factory lenes but you can always add a telescope with say 1200 mm - 2000 mm if you are ok w manual focus.
Those steady cam video systems on NG helicopters can shoot at 3200 mm equivalents.

To me, a longer telephoto would typically be less about long distances and more about magnification, so that the bird I'm shooting can be framed without the need for close cropping.

The times I've done longer distances have not been with 800mm but more in the 600mm zone -- a 300mm with a 2x and 400mm with a 1.4x. Sometimes I've battled haze, sometimes not so much. In fact, it can be sweet getting the moon on a clear night!

So, I wouldn't complain if someone gave me a 1200, although the support required would in itself probably cost an arm and a leg!

For a place like Yellowstone, well, I guess if I was free to take my pick I'd grab an 800, just because. I've never spent time looking at the archives for the 800, though. I do keep up with the 500, because, well, if I ever were to go for another supertelephoto that would have to be my pick because it is portable (which is a real good thing for the shooting I do) and it is super in its IQ. But, sadly, my finances will likely never allow me the luxury!