just got the tubes and went out in the BY for some testing. AF is a problem as it is very jumpy (in and out)...I will probably start MFing These are shot handheld in MP 1/640, f10, auto iso (some iso > 2500) minor cropping since i was so close. Overall 1st impression: for $169 (and free ship at b&h) it got me alot closer and IQ seems to hold up fine. I've only tried 1 tube (the biggest) and even with the 300 & TC it was sturdy enough (I still held the lens with left hand for support). Alot more testing will be necessary and I'm ready.