The other day I was experimenting again, and dug out the old Sigma 105mm 2.8 macro lens, my main macro provider until it quit unexpectedly about 16 years ago (!), in two ways. The first was that autofocus got very balky, likely due to a stripped focus gear, though I have no idea how that happened. I did indeed do this on my own with another lens, by inadvertently leaning the focus ring against a
