Wow, thanks for that detailed information Chuck. You guys are such a
valuable info resource.
I will give it a go over the weekend.
matt
On 20/11/2007, Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx> wrote:
>
> 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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--
Matt
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"It is not the strongest of the species that survive, nor the most
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