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Author Message   Glenn Tinkerer Username: Glenn Post Number: 793 Registered: 07-2006 Rating: N/A Votes: 0 Posted on Wednesday, June 30, 2010 - 03:15 pm:    By 'Canon original' I presume that your rear cap does NOT have finger grip serrations around the rear diameter? On this cap the three cap lugs are actually tapered wedges - in use the cap is prevented from rotating in the mount by a slot in the lug fitting over the lens locating pin, turning the breech ring jams the alloy ring onto the wedge sectioned cap lugs 'locking' it in place and preventing accidental rotation of the breech ring. When the cap has been positioned properly on the rear of the lens you can see equal amounts of all three lugs - obviously all the lug before rotating the breech ring and approx half of each lug when an unworn cap is secured. Your problem arises because of two things - a/ over the years the wedge shaped lugs wear ever thinner and this eventually allows b/ the cap does not positively fit over the locating pin and rotates as you move the breech ring. Invariably this leads to one of the cap lugs over riding the location pin and jamming the breech ring solid. One should note that there are some new rear caps for breech lenses available on eBay etc, these are all correctly marked up Canon but unfortunately the lugs are not wedge shaped - obviously the pattern maker used a worn cap as his sample - these caps will lock up immediately they are used if not put on carefully! To remove the cap you must hold the cap tight and turn the breech ring so that it rotates off the cap lugs - clockwise when viewed from back of lens. Ideally you need a couple of the plastic and rubber strap wrenches used to remove stuck filters. However you can make a couple of strap wrenches from a couple of length of 1/2"x1/2" hard wood and an old leather/plastic belt. A very small drop of thin oil placed at the interface of lug and breech ring can make things free up easier, from your description it appears that only one lug/ring interface is visible - obviously when the jam is not too severe there will be three interfaces to lube. Apply the drop of oil from the tip of a sewing needle. ( Purists should note that I always use decanted WD40 as my lubricant of choice for this type of job and I have come across a fair number of ham fisted collectors over the years!). Your comments about the diaphragm and focus rings are superfluous - they have no bearing on the problem what so ever.