Thought I'd share this, as I'm going through a lens repair in the MF thread.

I found that none of the tricks (rubber plug, rubber gloves, rubber bottle opener) could get a retaining ring to budge. This teardown's more of a learning experience and I didn't want to order a spanner and wait a week to get back to this. Two grip heads, two screwdrivers, and a 12" piece of 5/8 boom arm and... spanner!