The "correct" fix is total disassembly, cleaning and re-lubing of the
helical. However, you can often get away with swabbing the blades with
Ronsonol lighter fluid and then working them (or trying to) until the
solvent evaporates carrying the old lube with it. You may have to do it
a dozen or otherwise many times before it's cleaned enough that it
starts to operate. Ronsonol is specified here because it's pure naptha,
works well and comes in a can holding a small quantity and also has a
convenient pin hole spout.
Whether you go in from the front or the back depends on the lens. Some
of the 50/1.8s have thread lock on the name ring which can be difficult
(and sometimes impossible) to break loose. As Moose says, the first
thing to do is to remove the mount and linkage so see if damage or dirt
is the problem there. Since you're already part of the way there I
would make the next step to see if unscrewing the rear element cells
opens everything up to reveal the back side of the diaphragm. It may or
may not. If it does you're most of the way home. Just drip Ronsonol on
the blades a drop or two at a time and work the blades while being
careful not to let it run through and get on the lens elements in front
of the blades. If the blades are stuck open and can't be operated the
only thing you can do is repeatedly drip Ronsonol around the periphery
of the opening and let it slowly evaporate. Eventually the blades will
free up and start to move. If you can't easily get to the blades
through the back you'll have to start from the front. Acetone (nail
polish remover) can dissolve thread lock but it also removes paint.
A very old note from John H on the 28/2.8 says that the most likely
culprit on this lens is dirty/greasy linkage and not the diaphragm so
definitely start at the back. I think the instructions to go in through
the front assume that you are going to remove the blades for cleaning. I
try to avoid that altogether.
Chuck Norcutt
Chuck Norcutt
Moose wrote:
> NSURIT@xxxxxxx wrote:
>> If I have something worth fixing it goes to Camtech, however I just received
>> a 50mm f1.8 and the aperture blades will not move on at all and also a 28mm
>> f2.8 that has slow aperture blades. Neither of these are lens upon which I
>> would pay to have a CLA. Any words of advice from those who have traveled
>> this road about traps to be aware of when opening these guys up? What method
>> works best for getting the blades cleaned and working. I might mention the
>> the 28mm doesn't appear to have any oil on the blades. They are just
>> really slow. Bill Barber
>>
>
> The first place to look is always under the mount, especially for lenses
> other than the 50/1.8. The DOF mechanism is pretty simple and
> straightforward, but it can get dirty, a plastic pin in a few models can
> distort or break, etc. It's also easy to move it and watch whether it is
> what's slow or the aperture mechanism deep inside.
>
> You can easily remove the rings and springs and operate the aperture
> with a small screwdriver or other probe. If the problem is indeed
> inside, on the 50 and probably on the 28, you go in from the front.
> although the name ring probably has notches for a whatsitcalled spanner,
> and I've been able to open them that way, I gather that a rubber stopper
> using simple push hard and twist may be better. Apparently bathtub
> stoppers fit some smaller lenses well, or one can go with one from a lab
> supply house or the on line camera repair supply place.
>
> Specifically as to the 50/1.8, the problem of oil on the blades and in
> the mechanism is particularly prevalent in the ones marked "MC",
> although I had one "miJ" with a modest problem. In the MCs, cleaning the
> blades alone may be only a temporary solution. I suspect the only long
> term solution is to clean the whole mechanism, removing and replacing
> the helix grease. Still, the simple cleaning is pretty quick and easy,
> and may be repeated.
>
> Moose
>
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