Moose has described it fairly well. The center ring is nothing but a
cover. Removing it will allow you to see the mechanisms better. The
long diaphragm actuating rods that Moose describes are attached to the
outer mount ring and don't come out except by removing the mount ring.
Before you remove the ring set the diaphragm ring to f/16 (or smallest
setting) and watch what happens with the control rod inside. Then
rotate the diaphragm actuating lever to see what happens. In the case of
your lens, probably nothing. But what's supposed to happen is that the
long actuating rod rotates around out of the way and allows the
diaphragm spring to close the diaphragm down as far as the aperture
setting allows.
Hopefully, you will see something obvious at this point and be able to
correct it by a little bit of judicious pushing or bending as Moose
desribes without having to remove the mount ring.
As to removing the mount ring, if you understand what's happening before
removing it and also the relative positions of the parts (important)
then getting it back together after pulling the mount ring off shouldn't
be too difficult. But if you didn't find something obvious before
pulling the ring the likely cause of your problem is oily diaphragm
blades. *Proper* cleaning is a big task requiring major disassembly.
*Improper* cleaning can *sometimes* be accomplished by unscrewing the
rear lens group to gain access to the diaphragm from the rear. Then,
using something that can carry one small drop of naptha (Ronsonol
lighter fluid in handy dispenser can recommended), let the drop of
naptha be drawn by capillary action into the edge opening of the
diaphragm cage where the blades are stuck inside. Rotate the lens to a
different position and repeat. Then, after you've done this somewhere
between 10 and 1,000 times you may find that the naptha has finally
allowed the diaphragm to s-l-o-w-l-y close. You won't see much rapid
action at this point since even the naptha on the blades is a major drag
on the teeny-tiny diaphragm spring.
Getting the blades to close is a major victory since you can now more
easily get more naptha on the blade surfaces and you can begin to work
it in by actuating the blades... slow as they are. Repeat again
somwhere between 10 and 1,000 times. What you're trying to do in all of
this is to get the naptha to dilute the oil and then to evaporate away
carrying the oil with it.
Cautions: Hold the lens fairly well parallel to the ground when doing
this. You don't want drops of naptha falling onto the rear of the front
lens elements on the other side of the diapragm. Make sure your
screwdrivers are tight fits in the screw slots on the back of the lens.
These screws may have threadlock on them and may be a bit of a challenge
to loosen. You don't want to gouge up the screw heads.
Finally, if you accomplish all this improper cleaning you may find that
the problem is back next year or next month since the real problem is
caused by focusing helicoid grease which has lost its viscosity and is
slowly migrating elsewhere in the lens. The correct fix is complete
disassembly, cleaning off all helicoid grease and regreasing the
helicoid with stable stuff.
Chuck Norcutt
Moose wrote:
> Don Holbrook wrote:
>
>> Long time, quiet (till now), digest reader. Have a 24-48/3.8 lens
>> that the diaphragm will not stop down. f-stop ring & rear tab move
>> ok, stopdown button moves rear tab ok Assume either stuck or
>> linkage problem. Can anyone point out what to look for if I pull
>> the center ring off the rear mount that the tabs come thru, or lead
>> me to a parts diagram that will show the linkage. Thanks in
>> advance. Off-list OK.
>>
>
> I don't know specifically about this lens, but there are a lot of
> similarities in those of various brands that I've had apart. Since
> the lens elements move as it is focused and/or zoomed, the mechanism
> that stops it down is generally some sort of long metal thingie that
> sticks deep down into the lens from the aperture stop down ring on
> the back and engages with some sort of tab on the diaphragm
> mechanism. Sometimes it is like a two pronged fork that engages the
> tab, sometimes ust a lever that pushes against the tab from one side.
>
>
> I've dealt with a few lenses where this mechanism has come uncoupled,
> with the lever/fork slipping off/past the tab. Usually, well, every
> time so far, it has been possible to get it right easily. Not a bad
> idea to see if it needs a small amount of judicious bending towards
> the center so it will stay on in the future. The mechanism that
> controls the tab that conveys aperture setting to the camera body is
> similar, but so far always a fork, since it must positively locate
> the tab against pressure in both directions.
>
> Take a close look at how the ring assembly is assembled before
> pulling out. You need to be able to get all the bits in the right
> places with the connections down into the lens body connected
> correctly all at once. On some lenses, it's easy, others are more
> challenging, easier perhaps with more hands. :-)
>
> Moose
>
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