Thank you so much for detailed explanationThank you very much, Imago. Much appreciated!
It is early days yet and at this stage it is hard to pick between them IQ-wise when the 600TC is at 840mm. The 600 TC does have better AF ability. The 800 is lighter and is a little easier to maneuver but not a big deal. I didn't sell the 800PF for any other reason other than I needed to help fund the 600TC but I really didn't need the 800PF once I had the 600TC. In other words, I didn't get rid of it due to any IQ issues, it was purely financial and would have been a double up with the 600TC. I loved the 800PF whilst I had it but now I have the 600TC I won't miss it.
The reason for getting the 600TC were a number of factors: I wanted a pro lens to replace my 400 f2.8E FL VR but I had to sell it to fund either a 400TC or the 600TC. I couldn't justify the $25,000 asking price when I would only get $8-9,000 at best for my 400 f2.8E FL VR as a private sale. I would have to advertise it in the second hand columns and deal with buyers and cash and all that rigmarole - I just kept putting it off. A special deal came up where I was able to snag $10,000 for the 400 f2.8E FL VR and a great deal on my 800PF as well. The decision was between the 400TC and 600TC. I found I was mostly shooting at 600mm+ and I mostly used the 800PF and thus I would just about always be using a 400TC with at least the internal TC and also with the other 1.4x TC or a 2x TC for 800mm. Thus the 600TC it was. When I had the 800PF I needed a shorter lens as well and I used the 500PF - this meant swapping lenses. I may even take a 70-200 f2.8 or 100-400 but an 800PF, 500PF and either the 70-200 or 100-400 would all take up much space in a backpack. The 600TC alleviates that, I can just use it by itself or I can add the 100-400 or 70-200.