I haven't read all the comments above, so this may be redundant, but the two lenses really don't seem comparable at all. I have the 800PF. I will say, it's a bit of a bear in comparison to my 500PF to routinely get on critters quickly. So if a 'birder' really wants to take advantage of the 800PF, it likely had better be stationary birds or birds that are in the distance. It's a tough, no go, for things like warblers, etc. that are on the constant move in thick stuff. Taking the 800mm out for slower moving, larger mammals is cool, but in the parks where I can shoot it often proves to be too much lens and I would miss MANY good shot opportunities with just the 800mm. Also, atmospheric conditions get in the way of that reach awfully quick as the sun comes up depending upon what terrain you're shooting over, how far away is the subject, the background, etc. I've taken some decent photos with the 800mm, but am really finding its sweet spot (FOR ME) to be early morning videos (talking first light type stuff) where the ISO for photography would make me unhappy with the results. It really shines for video clips IMO. For most birding work, I honestly thing a good 600mm would be superior. The 200-600mm would bring great utility if on a walk about without a specific target or plan. If there was a 600mm PF, I'd trade the 800mm PF for it in a second. Just my views.