Until Olympus builds a serious constant aperture telephoto zoom ......

[Reflector]

I just bought a pre-DG version off eBay... All I can say is WOW. The birds were not cooperating with me on this overcast day. It'll take a 1.4x and 2x TC (Sigma TC-1401 and TC-2001 respectively) very well if you place those in front of the Speedbooster to neutralize it out as a 1x and 1.4x but I don't think the pre-DG version is good enough to handle a 2x+1.4x(Kenko non protrusive Pro 300) combo. It tends to get soft even when stopped down. On the flip side: An internally zooming 240-600mm.

IBIS will struggle for sure, it'll handle the vibrations but it doesn't really "stabilize" the EVF like optical stabilization because it runs out of travel fast at 300-600mm. Technically hand-holdable at 1/200s or even lower with that crazy 600mm setup using the TC stack (Or 2x+plain tube)

But as a 85-210mm f/2... That's a very unique lens. Thanks for the MTF charts on it Jim, I'm already enjoying this lens but I need my subjects to actually fly out and feed on nectar instead of hiding in the trees.

Jim has a table on the specs, but the non OS version is 5.7lbs instead of the 7.5lbs monsters that come later. Definitely "handholdable" if you felt like using it that way. I know I like using it that way but I use an adapted Sigma 50-100 myself...