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Author Message   Gez Tinkerer Username: Gez Post Number: 166 Registered: 09-2007 Rating: N/A Votes: 0 Posted on Thursday, December 24, 2009 - 01:49 pm:    It's oil. The FD 50mm/1.4 made about 1977 are notorious for this problem. The oil doesn't evaporate, it merely spreads out onto the blades with increase in ambient temperature. There is lots in the archive on how to clean the 50/1.4 iris assembly, it comes out as a unit for easy cleaning. Removing the front optical cell may prove difficult, the use of a flexiclamp, either home-made or bought, is recommended. Before removing the iris unit measure the diameter of the aperture at F16.