While it's possible that something is broken in the linkage I think it's
far more likely that there is oil on the aperture blades. Many Japanese
built lenses of the 70's and 80's suffer from age related breakdown of
the high viscosity grease used to lubricate the focusing helicoid. The
grease, which should normally be about the same viscosity as automobile
wheel bearing grease, starts to turn into a thick oil and begins to flow
very slowly inside the lens. Eventually, the oil may reach the aperture
blades. The blades should be bone dry. If they get a bit of oil on
them they'll stick to each other rather tenaciously. The thing that
closes the blades is the tiniest of springs and it is unable to overcome
the sticky oil. If an OM mount lens is left in storage for a long time
it will likely be stuck in the full open position if the helicoid grease
breaks down and the oil starts to flow.
First, do a test to see if you can make the blades move at all. Install
your 4/3 adapter on the 50mm. We'll use the 50mm first to test that
your adapter is OK. The 4/3 adapter should have a small pin or arm that
extends radially inward. This pin serves to push the diaphragm lever to
the closed position just like an OM camera body does when the shutter is
fired. With the adapter installed on the 50mm lens and the lens held in
your hand (not on the camera) you should see the blades open and close
as you turn the aperture ring. If not there's something wrong with your
adapter or your 50mm.
Assuming that the 50mm and adapter are working OK together repeat the
test with the Sigma lens. Presumably nothing is going to happen because
the blades are stuck. Next, with the adapter still installed, set the
aperture ring to the smallest setting such as f/16 or f/22. Then hold
the lens with one hand and slap it hard into the palm of your other
hand. You're trying to give it a hard but cushioned blow. Sometimes,
giving it a good jolt will cause the blades to close down a bit. Check
to see if there's any movement. If not, try it another half dozen times
or until your patience gives out. If the blades do move it's possible
that repeated opening and closing will free them up.
If you see no movement or only a small amount it's likely that the
required solution is cleaning any oil off the blades. Properly done,
this requires a complete disassembly of the lens, cleaning the flowing
oil off all internal surfaces and a regreasing of the helicoid. But
there is a simpler fix although often only temporary. By removing
either the front or rear lens group you may be able to gain access to
the aperture cage. I would try the rear group first since you'll want
to remove the lens mount anyhow just to check if there is something
wrong with the diaphragm linkage. That usually only involves removing
the three screws on the back of the mount. Caution: If removing the
mount also allows removing the diaphragm ring be careful. There may be
a teeny-tiny spring and detent ball under the diaphragm ring. Don't
lose it. Once the mount is removed it should be apparent how to unscrew
the rear lens group. A spanner or some other tool may be required and
the lens group may also be installed with some thread lock. Check the
threads for shiny looking "glue". If present it may require a drop or
two of acetone to dissolve the glue. Careful: acetone (finger nail
polish remover) dissolves paint.
Once the rear group is removed you can stick a tool inside to drip-drop
cigarette lighter fluid (naptha) onto the edge of the diaphragm cage
where it can flow into and over the blades. Hold the lens horizontally
while you do this so you don't drip naptha through the aperture and onto
the back side of the front lens group. You'll have to use some
ingenuity as to the tool used for carrying a drop or two at a time.
Even a piece of heavy wire might work to carry and transfer a drop on
the end of the wire. Or a pair of tweezers with the tips held closed
and then opened once inside... or an eye dropper or a tiny plastic tube
such as used on spray can nozzles.
If the problem is oil then eventually the highly volatile naptha will
dissolve it and carry it away as the naptha itself evaporates. It takes
patience. You may have to do this 100 times. Also try the palm
slapping trick during the process. If you can get the blades to start
moving and poking out a bit you can start dripping even more lighter
fluid on them and begin to get them working a bit which will speed the
evaporation process.
Good luck.
Chuck Norcutt
Mark Marr-Lyon wrote:
> Yes, it looks like a Sigma lens. 'Pantel' seems to indicate that it
> stops down to f/64, though I'd think diffraction would cause it to be
> pretty soft there. Often you can fix the aperture by taking the back
> mount off and seeing where the linkage is disconnected. You need some
> small screwdrivers and a bit of dexterity. Not having an aversion to
> possibly ruining the lens also helps :) As others have said, it's
> probably not worth paying someone to fix it, but it can probably be
> pretty easily repaired.
>
> For OM mount lenses, only one button is needed to remove the lens.
> The other button is the DOF preview button, which as you have found
> stops the lens down so you can see what effect the aperture setting
> has.
>
> Mark
>
> On Nov 19, 2007 9:49 PM, Matthew Granger
> <matthewcharlesgranger@xxxxxxxxx> wrote:
>> I think it is Sigma brand, and Pantel is the model or version... but not
>> sure.
>
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