I recently bought a refurbished Mk3, and for the most part, the camera has performed just like it's supposed to. I use it primarily with a 100-400, and the shots I take with that lens are sharp and crisp. My other body is a 40D and the 100-400 performs equally as well. My primary "long distance" lens is a 300mm f2.8L IS, 99% of the time with either a Canon 1.4x TC or a Canon 2x TC. With the 40D, the lens is incredible! Very sharp images across the board, regardless of with or without TCs. On the other hand, the 300 with the Mk3 is less so, and some shots I made yesterday with the Mk3 and 300+2x were clearly disappointing. I figured I'd use the microadjust feature in the Mk3 to fine-tune the 300 to the camera but want to know if that adjustment should be with or without TCs... my guess is that the adjustment should be made without... Can anyone weigh in on this?


