Originally posted by Adam You also need to nail the focus.
This is by far the biggest challenge, I'd say. Longer lenses let you scale the image down while still getting a big view.
I went out the other night and played around with my DA*300 + 1.4x TC and 2x TC. The 2x TC will not autofocus with the DA*300, so nailing the focus is like trying to hit a pinhead with a syringe from a few feet away. Sure, you can hit it ok, but everything else gets wet too.
My experience was best with the 300 and 1.4x TC. Autofocus still works, so it came out the best. It wasn't razor sharp, but it wasn't bad.
For the fun of it, I stacked the 2x and 1.4x to make for a 840mm lens. Image degradation make the end result not terribly much better than with a single TC.
It's clear again tonight, so I think I'll take the Sigma 70-200 with the TCs out. I think the focus may be a bit easier to nail.