(Re: Mike W.'s originating query.)
I'm guilty of using the term 'snappy' in lenspeak to mean crisp, sharp,
contrasty, etc.
The problem you described is usually due to a lazy diaphragm return spring,
dried lubricant (and the dust sticking to it), or both. It can be repaired.
The question is whether the lens is worth the time and trouble, or
expense, to you. For a Vivitar I'd say, probably not. For a Zuiko 50/1.2
or 55/1.2, I'd say, heck yeh!
I've repaired my own Canon mount breechlock lenses. It isn't difficult,
just very tedious and one must be careful to note where all the springs and
screws came from. I haven't, and probably wouldn't, attempt the same with
later Canon pushbutton mount FD lenses. I might be wrong but a cursory
examination tells me there are snakes and rabid wolverines lurking inside
that design.
The OM lens looks not terribly difficult to dismantle and reassemble. But
I'd advise practicing on something like a 50/1.8 first.
If you do decide to take the plunge, I'd also suggest using dry molybdenum
disulphide powder (resembles graphite powder) on the diaphragm. Worked well
into the degreased blades, it'll last forever without causing future
problem. Shake or gently blow and/or vacuum out the excess.
For focus and zoom rings I like a thick, sticky white translucent goo sold
by Beeman Airguns under the name Spring Dampening Compound. It has good
enough lubricity for use in a lens and will never, ever run or migrate.
Beeman also carries dry moly and other lubes.
For springs, I'd cannibalize a similar spring from a junker lens or
teleconverter.
Most of the OM mount aftermarket lenses I've seen in pawn shops lately have
problems similar to what you've described. Often the diaphragm stops down
when the aperture ring is turned rather than when the DOF preview button is
depressed. None of 'em was worth messing with, other perhaps than for
parts.
Lex
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