With oil on the blades I would normally expect a progressive decline
starting with overexposed images caused by a sluggish and slow to close
down diaphragm. I have had some success treating such lenses by
removing the mount, linkage and rear elements and slowly dripping
Ronsonol lighter fluid (naptha) inside the diaphragm cage to evaporate
the oil. However, I once had a Kiron 35-135 that was stuck fast, gave
no signs of oil and couldn't be revived with Ronsonol. When I took it
apart I found a tiny piece of black grit (possibly a paint chip) wedged
between the diaphragm blades retaining cage and one of the blades.
Since they're all tied together they were all frozen. After removing
the bit of grit I noticed the blades were still sluggish. The clearance
between blades and cage wall was almost non-existent and I think one of
te blades wasn't perfectly flat and was dragging. I stuck a shim of
about .002" thickness between the sides of the diaphragm cage and then
the diaphragm worked perfectly.
I've never had a 24/2.8 completely apart so I don't know if there is
some likely common cause there. Maybe JH or Mark has some guidance.
Chuck Norcutt
Tom Fenwick wrote:
> It does sound like it could be that then. Do they generally become
> completely stuck with no warning, or would you expect to observe a
> progressive decline?
>
> Tom
>
> On 28/02/07, Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx> wrote:
>> The fact that it moves freely and feels right doesn't matter. On an OM
>> mount lens there is no physical link between aperture lever and
>> diaphragm except for pushing the aperture back open. On stop down the
>> lever just gets out of the way to allow the blades to close and a very
>> weak little spring pulls the blades closed.
>>
>> When you activate the stop down lever you're still feeling normal spring
>> tension for the stop down lever itself... whether the diaphragm follows
>> along behind or not.
>>
>> The simplest analogy I can draw is of a pinball machine with it spring
>> loaded ball release and the ball. In the analogy the spring loaded ball
>> release is the stop down lever inside the lens. The ball is the
>> diaphragm and gravity in combination with the slope of the table is the
>> lightweight diaphragm spring. When you pull back the ball release you
>> expect the ball to follow along behind due to gravity. If it doesn't
>> maybe there's some sticky substance under the ball preventing free
>> movement. That's oil on the blades.
>>
>> Chuck Norcutt
>>
>>
>> Tom Fenwick wrote:
>>> I don't think that's what it is - everything moves freely, the spring
>>> tension feels right etc, but the iris just doesn't stop down! It was
>>> working fine last time I tried it, about a month ago...
>>>
>>> Tom
>>>
>>> On 28/02/07, Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx> wrote:
>>>> What's the probability that this thing has oil on the blades and is
>>>> stuck wide open?
>>>>
>>>> Chuck Norcutt
>>>>
>>>> Tom Fenwick wrote:
>>>>> Hi,
>>>>>
>>>>> I've got a 24/2.8 which has stopped stopping down.
>>>>>
>>>>> I've removed the easy part of the back of the mount, and everything in
>>>>> that section is working ok; ie the lever on the main part of the lens
>>>>> is being operated but it's not doing anything.
>>>>>
>>>>> I guess something has come unhooked.
>>>>>
>>>>> Is it likely to be something I can get at and re-hook fairly easily?
>>>>>
>>>>> Thanks,
>>>>>
>>>>> Tom
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