Nikon FM Mirror/Shutter Problem

One of my FMs will not fire with my Vivitar or Sigma lenses. I don't think the mirror is flipping up properly, but when I inspect the mirror box it looks just like my other FM, which fires fine. The trouble camera works fine with Nikon lenses. I cannot see anything wrong, and the mirror appears to be seated properly, so I cannot figure it out at all.

Has anyone else had this issue?

Using a cable release lock the shutter open on B, attach a Vivitar or Sigma lens then release the shutter. Does the mirror close fully or does it stick on the back of the lens?

Thank you, I tried it and wow - it closed. I really expected that it would not. I then tried to fire it again immediately with the lens still on, and it didn't work. Sounds like the mirror moves a little (doesn't fully "clunk"), and I can no longer see anything through the viewfinder.

After it misfires like this, I expect that once I remove the lens it will automatically complete its cycle, but it doesn't with these two lenses. It is still cocked and will fire normally with the lens removed if I go ahead and press the shutter.

I just tried another third party lens (a cheap AF Tamron), and it fires fine. And then I tried a newer Nikon 50mm AFD, and it doesn't fire. But! - With the 50mm AF, it automatically completes its cycle when I remove the lens, without me having to press the shutter release. So weird. All these lenses work fine on my other Nikons (FMs, FEs, etc.)

I guess I need to send it in to a professional...

Is there any up-down or left-right play with the lens that the shutter doesn't work mounted?
Is there and rotational movement, if so is is more than when the lens is mounted on a working body?
Does the lens lock pin extend the same distance from the lens mount on all bodies?
Is the pin bent on the defective body?

I wonder if it's shutter speeds that are highlighting the problem, and maybe it's just a coincidence it happens with your Sigma and Vivitar lenses. A fast shutter speed maybe snaps the mirror up and down and it works ok, but a slightly slower shutter speed allows the mirror to grab on a sticky and degraded foam mirror bumper pad. Have a look at the foam and see if it's either flattened and compressed, or sticky, or both.

If not that then it's easy to do some simple lube work on the FM as most of the parts in the shutter mechanism that get dry are right under the baseplate, so a couple of screws will remove it. There are plenty of vids on YouTube on how to do it.

The lenses mount firmly, with no wiggle or rotation. The lens lock pin looks the same on all bodies, and releases smoothly. I really can see no difference.

The mirror sticks, regardless of shutter speed. I was under the assumption that the mirror acts the same regardless of shutter speed, but I tried it anyway and there was no difference.

Ariston said:

The lenses mount firmly, with no wiggle or rotation. The lens lock pin looks the same on all bodies, and releases smoothly. I really can see no difference.

The mirror sticks, regardless of shutter speed. I was under the assumption that the mirror acts the same regardless of shutter speed, but I tried it anyway and there was no difference.

Click to expand...

I think you misunderstand. There is a foam rubber pad that acts as a break for the mirror, so it doesn't just crash into metal when it flips up. If the foam that forms the mirror stop has degraded and gone sticky it will make the mirror stick to it when the mirrors flips up. The mirror doesn't act differently with different shutter speeds but the longer it is 'up' it has longer to get stuck to the mirror pad. But it was only an idea.

250swb said:

I think you misunderstand. There is a foam rubber pad that acts as a break for the mirror, so it doesn't just crash into metal when it flips up. If the foam that forms the mirror stop has degraded and gone sticky it will make the mirror stick to it when the mirrors flips up. The mirror doesn't act differently with different shutter speeds but the longer it is 'up' it has longer to get stuck to the mirror pad. But it was only an idea.

Click to expand...

Oh, sorry, I wasn't clear. The mirror is not going up in the first place. When I say it sticks, I mean it is sticking down, as though the lens is obstructing it. I can take the lens off and fire the shutter (without re-cocking) and it fires normally. The mirror is moving slightly, though, because the viewfinder blacks out. But from the feel of it when I release the shutter, it moves very little. Also, as I said, it consistently works on some lenses, and consistently doesn't work on these two. It is weird.

A suggestion: perhaps something in the lens mount or the camera's aperture lever is bent or out of position, so that with the problem lenses the camera's aperture link cannot move down fully. Obstructing the aperture link will pause the cycle, preventing the mirror from rising fully and the shutter from firing. (You can verify this by taking off a lens and pressing the shutter while carefully preventing the link lever from moving with your fingertip.)

I would guess that if this were the case, the shutter would fire immediately when the lens is removed, rather than you having to press the button again. However, if the camera's lever is bent, that could cause more drag. They sometimes get bent if a person tries to mount or remove a lens the conventional lefty-loosey way, rather than the reverse Nikon way.

reddesert said:

A suggestion: perhaps something in the lens mount or the camera's aperture lever is bent or out of position, so that with the problem lenses the camera's aperture link cannot move down fully. Obstructing the aperture link will pause the cycle, preventing the mirror from rising fully and the shutter from firing. (You can verify this by taking off a lens and pressing the shutter while carefully preventing the link lever from moving with your fingertip.)

I would guess that if this were the case, the shutter would fire immediately when the lens is removed, rather than you having to press the button again. However, if the camera's lever is bent, that could cause more drag. They sometimes get bent if a person tries to mount or remove a lens the conventional lefty-loosey way, rather than the reverse Nikon way.

Click to expand...

You may be onto something. The aperture lever looked fine to me, and I pushed it down manually and the "springiness" felt the same as on my other Nikon. However, looking very closely, it may be bent back ever so slightly... or my eyes may be playing tricks on me. I am going to take the ill-advised step of trying to bend it back toward the front and see if that changes anything.

Ha! You are a genius, reddesert, thank you! It was almost unnoticeable, but sure enough that lever was ever so slightly bent backward. All is well now!

Thank you to everyone for your input and help!

Great!