Originally posted by CptVoltarg I guess my questions are: does the 16-50 normally behave like this? Is this a sign of the dreaded SDM failure? How can I confirm if it is?
It's not about the lens itself, this is an inherited issue with wider focals, hence the focusing difference between the 50~135 and the 16~50. Looking at your "(f2.8, 1/40, ISO 3200)" note, that is pretty poor lighting and with a wider focal length contrast points are spread thin (meaning the appearance of quite a bit smaller, etc) and thus focusing is a little trickier as there is a very very thin contrast point for the AF system to lock on and the slightest movement of the body effects the locking on of that particular contrast point.
To give you an example, take your 50~135 and focus on something at the minimum focusing distance at around 75mm, then take your 16~50 and focus on that same spot at 16mm at about the same distance then compare the two, there is a much bigger contrast point in the 50~135 shot as opposed to the 16~50 shot.