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!