I have a question for the 500+ guys - how many of you shoot bare glass (i.e. with no extenders?). I was reviewing Art Morris' blog posts the other day and noticed that he almost always has an extender (usually the 2x!) on his 600mm. I realize he has a style that requires close shots, but it mirrored my experience with the 800mm. It just didn't seem long enough - or should I say it was too long for stuff that was close to mid-distance but not long enough for nesting eagles and such.
600 no extender = 95%
600 with 1.4iii = 5%
600 with 2xiii = 0%
300 no extender = 50%(sports mostly)
300 with 1.4iii = 40% (Sports mostly)
300 with 2xiii = 10%(birds on walkabout but bright sun only like on a beach)
The bottom line is that I do not find that adding the 2X buys a lot. I often find I can crop the 5diii and get better IQ. If the subject is too far away that you need the 2X then you probably should try something else. I speak for at least what I shoot most, large to medium size birds and sports looking for the highest IQ possible to minimize post efforts.