newFD lens, strange behaviour of diaphragm

AgX · May 7, 2017

I got me a mint Canon newFD lens. I played a bit with it, all worked fine, including the DOF operation.
Suddenly that DOF operation stopped to work. The aperture no longer closed in manual setting. The diaphragm apparatus though was working. With the aperture set at 16 and then taking off the lens, the aperture closed to 16 for a moment during twisting, as expected. Repeated taking on and off the lens, in whatever mode though did not make the DOF function work again.
Yes, the DOF switch always was on idle when putting on the lens. And with the lens off the aperture actuator worked perfectly when pressing the DOF switch.
However when operating the lens by releasing the camera the aperture closed perfectly and when then operating the DOF switch all was fine again too!

I got NO explanation for this behaviour.
(Aside of course erratic behaviour combined with strange coincidence.)

cooltouch · May 7, 2017

Couple of questions: which camera are you using (different FD cameras have different stop down engagement mechanisms)? And I'm not clear on what you meant by "releasing the camera". Did you mean just hitting the shutter release?

And another question: When the iris was stopped down, did you inspect it for any traces of oil on the aperture blades? If there is a very slight amount of oil on the blades, this might account for the erratic behavior. If there is any more oil on the blades, then usually the aperture doesn' t work at all and stays in the fully open position.

AgX · May 7, 2017

Yes, hitting the release button.

The lens is mint except for this strange behaviour. And it hardly could not be explained by an oily diaphragm. And yes, I got oily FD lenses and know the behaviour of such diaphragm. With these lenses I got, all act the same, retarded but not erratic.

The camera was an AE-1.
FD-cameras all activate the diaphragm the same way. If you refer to the aperture control lever, I had set the lens on manual when I got the aperture to work on the body, so that lever was not involved, only the actuator for the automatic diaphragm.

And between trials I twisted the aperture ring from one end to the other.

I always thought I knew the FD-system well, that is why I am so puzzled.

flavio81 · May 7, 2017

AgX said:
I got me a mint Canon newFD lens. I played a bit with it, all worked fine, including the DOF operation.
Suddenly that DOF operation stopped to work. The aperture no longer closed in manual setting. The diaphragm apparatus though was working. With the aperture set at 16 and then taking off the lens, the aperture closed to 16 for a moment during twisting, as expected. Repeated taking on and off the lens, in whatever mode though did not make the DOF function work again.
Yes, the DOF switch always was on idle when putting on the lens. And with the lens off the aperture actuator worked perfectly when pressing the DOF switch.
However when operating the lens by releasing the camera the aperture closed perfectly and when then operating the DOF switch all was fine again too!

I got NO explanation for this behaviour.
(Aside of course erratic behaviour combined with strange coincidence.)

Which lens is it? Which camera?

It could be some stickiness inside the lens mechanism. Perhaps when you fire the camera, since the aperture setting lever AND the stop down lever are moving, the mechanism as a whole works smoothly. But when only actuating the stop down lever, diaphragm leaves do not move.

BTW when you take out the lens from the camera, the aperture moves via the aperture setting lever, not via the stop down lever. Your problem is clearly with the stop down lever.

AgX · May 7, 2017

AE-1 , 50mm 1.8

But the stop down lever (automatic diaphragm lever) is the same whether in DOF-mode or in release mode.
And yes, in contrast the aperture control lever would be the only variable. I shall think about it.

AgX · May 7, 2017

It could be I am an idiot...

After a lot of trials it turned out the closing of the aperture beyond 5.6 when taking off the lens only happens if before, at putting the lens on, the DOF switch was engaged. Something I was sure of I never ever did... (I use the FD system since the 70s.)

Then of course the actuating of that switch would be of no effect.
The same time it would mean I not only put the lens on with that switch set on erroneously, but also released it before actuating it again.
Hard to believe.
But who knows what idiots are capable of.

Anyway, that scenario was the only way I succeded in replicating all encounters.

Most embarrassing...

AgX · May 7, 2017

Still I prefer being an idiot than to realize to have a broken lens. So I hope it is the former.

Thread closed.

cooltouch · May 7, 2017

With a New FD lens, when it is rotated to release it from the camera, the iris stops down to a mid-level -- in the case of my 85mm f/1.8, this is f/8. Might be different with others, I don't know. My 85 is what I have handy at the moment. This is the default iris configuration with a New FD lens when not mounted to a camera. I don't recall, off-hand, what is the norm with the breechlock lenses. So anyway, if your 50/1.8 is a New FD lens, does it exhibit this stop-down behavior when dismounting the lens? If not, this would be a place to start in the investigation, I'm thinking. You mention you know the FD system well, so I'm probably not stating anything you're not already familiar with, but it never hurts to ask.

AgX · May 8, 2017

Thread opened again...

I'm rehabilitated.
It happened again.

flavio81 · May 8, 2017

AgX said:
Thread opened again...

I'm rehabilitated.
It happened again.

AgX,

The FD New 50/1.8 is trivial to dissasemble, so why don't you inspect the mechanism?

WIth the lens on top of a table, rear side facing up, remove the three small screws around the lens mount. They hold the (chrome) mount ring of the lens.

Now remove the chrome ring by pulling it up. Once removed you have the plastic part that rotates when the lens is fitted to the camera. Remove it carefully by lifting it up, perhaps wiggling a little bit.

Afterwards you will see most of the aperture mechanism. Take a picture to see where goes everything. Still, it is very easy.

Then you can take a look if something is getting sticky, either in the lens aperture mechanism, or in the mount you have just removed.

If you want to dissasemble further the next step is removing the three big screws that hold the rest of the base of the lens.

Greetings,
Flavio.

newFD lens, strange behaviour of diaphragm

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